Nightlies for pypy no longer run on Ubuntu 12.04; change the pypy build
distribution to the travis-ci trusty (i.e. 14.04) beta container.

The pypy build was also installing numpy and scipy for the *system*
python version, which was pointless; this also adds a guard to the
eigen/numpy/scipy install code with a !PYPY check.

When using pybind::options to disable function signatures, user-defined
docstrings only get appended if they exist, but newlines were getting
appended unconditionally, so the docstring could end up with blank lines
(depending on which overloads, in particular, provided docstrings).

This commit suppresses the empty lines by only adding newlines for
overloads when needed.

This makes array_t respect overload resolution and noconvert by failing
to load when `convert = false` if the src isn't already an array of the
correct type.

Remove extraneous `enum_` python constructor

Added in 6fb48490

The second constructor can't be doing anything--the signatures are
exactly the same, and so the first is always going to be the one
invoked by the dispatcher.

Expose enum_ entries as new "__members__" attribute

Commit 11a337f1 added major and minor python version
checking to cast.h but does not use the macros defined
via the Python.h inclusion. This may be due to an
intention to use the variables defined by the cmake
module FindPythonInterpreter, but nothing in the
pybind11 repo does anything to convert the cmake
variables to preprocessor defines.

* The definition of `PySequence_Fast` is more restrictive on PyPy, so
  use the slow path instead.
* `PyDict_Next` has been fixed in PyPy -> remove workaround.

Slightly reduces binary size (range for loops over tuple/list benefit
a lot). The iterators are compatible with std algorithms.

The added type aliases are required by `std::iterator_traits`.
Python iterators satisfy the `InputIterator` concept in C++.

Before this, `py::iterator` didn't do any error handling, so code like:
```c++
for (auto item : py::int_(1)) {
    // ...
}
```
would just silently skip the loop. The above now throws `TypeError` as
expected. This is a breaking behavior change, but any code which relied
on the silent skip was probably broken anyway.

Also, errors returned by `PyIter_Next()` are now properly handled.

Fixes test failure on Fedora 25.

Fixes some numpy tests failures on ppc64 in big-endian mode due to
little-endian assumptions.

Fixes #694.

test_eigen.py and test_numpy_*.py have the same
@pytest.requires_eigen_and_numpy or @pytest.requires_numpy on every
single test; this changes them to use pytest's global `pytestmark = ...`
instead to disable the entire module when numpy and/or eigen aren't
available.

This commit largely rewrites the Eigen dense matrix support to avoid
copying in many cases: Eigen arguments can now reference numpy data, and
numpy objects can now reference Eigen data (given compatible types).

Eigen::Ref<...> arguments now also make use of the new `convert`
argument use (added in PR #634) to avoid conversion, allowing
`py::arg().noconvert()` to be used when binding a function to prohibit
copying when invoking the function.  Respecting `convert` also means
Eigen overloads that avoid copying will be preferred during overload
resolution to ones that require copying.

This commit also rewrites the Eigen documentation and test suite to
explain and test the new capabilities.

Eigen::Ref objects, when returned, are almost always returned as
rvalues; what's important is the data they reference, not the outer
shell, and so we want to be able to use `::copy`,
`::reference_internal`, etc. to refer to the data the Eigen::Ref
references (in the following commits), rather than the Eigen::Ref
instance itself.

This moves the policy override into a struct so that code that wants to
avoid it (or wants to provide some other Return-type-conditional
override) can create a specialization of
return_value_policy_override<Return> in order to override the override.

This lets an Eigen::Ref-returning function be bound with `rvp::copy`,
for example, to specify that the data should be copied into a new numpy
array rather than referenced, or `rvp::reference_internal` to indicate
that it should be referenced, but a keep-alive used (actually, we used
the array's `base` rather than a py::keep_alive in such a case, but it
accomplishes the same thing).

Numpy raises ValueError when attempting to modify an array, while
py::array is raising a RuntimeError.  This changes the exception to a
std::domain_error, which gets mapped to the expected ValueError in
python.

numpy arrays aren't currently properly setting base: by setting `->base`
directly, the base doesn't follow what numpy expects and documents (that
is, following chained array bases to the root array).

This fixes the behaviour by using numpy's PyArray_SetBaseObject to set
the base instead, and then updates the tests to reflect the fixed
behaviour.

A few of pybind's numpy constants are using the numpy-deprecated names
(without "ARRAY_" in them); updated our names to be consistent with
current numpy code.

`is_template_base_of<T>` fails when `T` is `const` (because its
implementation relies on being able to convert a `T*` to a `Base<U>*`,
which won't work when `T` is const).

(This also agrees with std::is_base_of, which ignores cv qualification.)

Currently when we do a conversion between a numpy array and an Eigen
Vector, we allow the conversion only if the Eigen type is a
compile-time vector (i.e. at least one dimension is fixed at 1 at
compile time), or if the type is dynamic on *both* dimensions.

This means we can run into cases where MatrixXd allow things that
conforming, compile-time sizes does not: for example,
`Matrix<double,4,Dynamic>` is currently not allowed, even when assigning
from a 4-element vector, but it *is* allowed for a
`Matrix<double,Dynamic,Dynamic>`.

This commit also reverts the current behaviour of using the matrix's
storage order to determine the structure when the Matrix is fully
dynamic (i.e. in both dimensions).  Currently we assign to an eigen row
if the storage order is row-major, and column otherwise: this seems
wrong (the storage order has nothing to do with the shape!).  While
numpy doesn't distinguish between a row/column vector, Eigen does, but
it makes more sense to consistently choose one than to produce
something with a different shape based on the intended storage layout.

With the previous commit, output can be very confusing because you only
see positional arguments in the "invoked with" line, but you can have a
failure from kwargs as well (in particular, when a value is invalidly
specified via both via positional and kwargs).  This commits adds
kwargs to the output, and updates the associated tests to match.

Fixes #688.

My (commented) assumption that such an error is "highly likely to be a
caller mistake" was proven false by #688.

* Make tests buildable independently

This makes "tests" buildable as a separate project that uses
find_package(pybind11 CONFIG) when invoked independently.

This also moves the WERROR option into tests/CMakeLists.txt, as that's
the only place it is used.

* Use Eigen 3.3.1's cmake target, if available

This changes the eigen finding code to attempt to use Eigen's
system-installed Eigen3Config first.  In Eigen 3.3.1, it exports a cmake
Eigen3::Eigen target to get dependencies from (rather than setting the
include path directly).

If it fails, we fall back to the trying to load allowing modules (i.e.
allowing our tools/FindEigen3.cmake).  If we either fallback, or the
eigen version is older than 3.3.1 (or , we still set the include
directory manually; otherwise, for CONFIG + new Eigen, we get it via
the target.

This is also needed to allow 'tests' to be built independently, when
the find_package(Eigen3) is going to find via the system-installed
Eigen3Config.cmake.

* Add a install-then-build test, using clang on linux

This tests that `make install` to the actual system, followed by a build
of the tests (without the main pybind11 repository available) works as
expected.

To also expand the testing variety a bit, it also builds using
clang-3.9 instead of gcc.

* Don't try loading Eigen3Config in cmake < 3.0

It could FATAL_ERROR as the newer cmake includes a cmake 3.0 required
line.

If doing an independent, out-of-tree "tests" build, the regular
find_package(Eigen3) is likely to fail with the same error, but I think
we can just let that be: if you want a recent Eigen with proper cmake
loading support *and* want to do an independent tests build, you'll
need at least cmake 3.0.

* Make string conversion stricter

The string conversion logic added in PR #624 for all std::basic_strings
was derived from the old std::wstring logic, but that was underused and
turns out to have had a bug in accepting almost anything convertible to
unicode, while the previous std::string logic was much stricter.  This
restores the previous std::string logic by only allowing actual unicode
or string types.

Fixes #685.

* Added missing 'requires numpy' decorator

(I forgot that the change to a global decorator here is in the
not-yet-merged Eigen PR)

Now that only one shared metaclass is ever allocated, it's extremely
cheap to enable it for all pybind11 types.

* Deprecate the default py::metaclass() since it's not needed anymore.
* Allow users to specify a custom metaclass via py::metaclass(handle).

In order to fully satisfy Python's inheritance type layout requirements,
all types should have a common 'solid' base. A solid base is one which
has the same instance size as the derived type (not counting the space
required for the optional `dict_ptr` and `weakrefs_ptr`). Thus, `object`
does not qualify as a solid base for pybind11 types and this can lead to
issues with multiple inheritance.

To get around this, new base types are created: one per unique instance
size. There is going to be very few of these bases. They ensure Python's
MRO checks will pass when multiple bases are involved.

Instead of creating a new unique metaclass for each type, the builtin
`property` type is subclassed to support static properties. The new
setter/getters always pass types instead of instances in their `self`
argument. A metaclass is still required to support this behavior, but
it doesn't store any data anymore, so a new one doesn't need to be
created for each class. There is now only one common metaclass which
is shared by all pybind11 types.

* Switch breathe to stable releases. It was previously pulling directly
  from master because a required bugfix was not in a stable release yet.

* Force update sphinx and RTD theme. When using conda, readthedocs pins
  sphinx==1.3.5 and sphinx_rtd_theme==0.1.7, which is a bit older than
  the ones used in the RTD regular (non-conda) build. The newer theme
  has nicer sidebar navigation (4-level depth vs. only 2-level on the
  older version). Note that the python==3.5 requirement must stay
  because RTD still installs the older sphinx at one point which isn't
  available with Python 3.6.

[skip ci]

* Fixed compilation error when defining function accepting some forms of std::function.

The compilation error happens only when the functional.h header is
present, and the build is done in debug mode, with NDEBUG being
undefined.  In addition, the std::function must accept an abstract
base class by reference.

The compilation error occurred in cast.h, when trying to construct a
std::tuple<AbstractBase>, rather than a std::tuple<AbstractBase&>.
This was caused by functional.h using std::move rather than
std::forward, changing the signature of the function being used.

This commit contains the fix, along with a test that exhibits the
issue when compiled in debug mode without the fix applied.

* Moved new std::function tests into test_callbacks, added callback_with_movable test.

* fix warning when Python.h was included before pybind11.h

* remove trailing whitespace

* changed return_value:: to return_value_policy::

* Update functions.rst

noexcept deduction, added in PR #555, doesn't work with clang's
-std=c++1z; and while it works with g++, it isn't entirely clear to me
that it is required to work in C++17.

What should work, however, is that C++17 allows implicit conversion of a
`noexcept(true)` function pointer to a `noexcept(false)` (i.e.  default,
noexcept-not-specified) function pointer.  That was breaking in pybind11
because the cpp_function template used for lambdas provided a better
match (i.e. without requiring an implicit conversion), but it then
failed.

This commit takes a different approach of using SFINAE on the lambda
function to prevent it from matching a non-lambda object, which then
gets implicit conversion from a `noexcept` function pointer to a
`noexcept(false)` function pointer.  This much nicer solution also gets
rid of the C++17 NOEXCEPT macros, and works in both clang and g++.