Pybind11's default conversion to int always produces a long on Python 2 (`int`s and `long`s were unified in Python 3). This patch fixes `int` handling to match Python 2 on Python 2; for short types (`size_t` or smaller), the number will be returned as an `int` if possible, otherwise `long`. Requires Python 2.5+.

This is needed for things like `sys.exit`, which refuse to accept a `long`.

None of the three currently recommended approaches works on PyPy, due to
it not garbage collecting things when you want it to. Added a note with
example showing how to get interpreter shutdown callbacks using the Python
atexit module.

This changes the travis-ci eigen download code to extract the tar on the
fly (rather than saving to a file first), and extracts into an `eigen`
directory rather than using upstream's `eigen-eigen-xxxxx` directory.

This also bumps the travis-ci eigen release to 3.3.4, in an attempt to
see if it fixed the -Wdeprecated warnings (it did not); the build setup
cleanup seems worth committing anyway.

This commit turns on `-Wdeprecated` in the test suite and fixes several
associated deprecation warnings that show up as a result:

- in C++17 `static constexpr` members are implicitly inline; our
  redeclaration (needed for C++11/14) is deprecated in C++17.

- various test suite classes have destructors and rely on implicit copy
  constructors, but implicit copy constructor definitions when a
  user-declared destructor is present was deprecated in C++11.

- Eigen also has various implicit copy constructors, so just disable
  `-Wdeprecated` in `eigen.h`.

py::class_<T>'s `def_property` and `def_property_static` can now take a
`nullptr` as the getter to allow a write-only property to be established
(mirroring Python's `property()` built-in when `None` is given for the
getter).

This also updates properties to use the new nullptr constructor internally.

A few fixes related to how we set `__qualname__` and how we show the
type name in function signatures:

- `__qualname__` isn't supposed to have the module name at the
beginning, but we've been putting it there.  This removes it, while
keeping the `Nested.Class` name chaining.

- print `__module__.__qualname__` rather than `type->tp_name`; the
latter doesn't work properly for nested classes, so we would get
`module.B` rather than `module.A.B` for a class `B` with parent `A`.
This also unifies the Python 3 and PyPy code.  Fixes #1166.

- This now sets a `__qualname__` attribute on the type (as would happen
in Python 3.3+) for Python <3.3, including PyPy.  While not particularly
important to have in earlier Python versions, it's useful for us to be
able to extracted the nested name, which is why `__qualname__` was
invented in the first place.

- Added tests for the above.

Non-user facing. 
Found using `codespell -q 3`

The just-updated flake8 package hits a bunch of:

    E741 ambiguous variable name 'l'

warnings.  This commit renames them all from `l` to `lst` (they are all
list values) to avoid the error.

- For the debian/buster docker build (GCC 7/C++17) install and use the
  system `catch` package; this also renames "COMPILER_PACKAGES" to
  "EXTRA_PACKAGES" since it now contains a non-compiler package.

- Add a status message indicating the catch version being used for
  compiling the embedded tests

- Simplify some bash code by using VAR+=" foo" to append (rather than
  VAR="${VAR} foo"

- Fix CMAKE_INCLUDE_PATH appending: it was prepending the ':' but not
  the existing $CMAKE_INCLUDE_PATH value and so would end up with
  ":/eigen-path" if CMAKE_INCLUDE_PATH was already set.  (This wasn't
  bug that was actually noticed since currently nothing else sets it).

Building with the (VS2017) /permissive- flag puts the compiler into
stricter standards-compliant mode.  It shouldn't cause the compiler to
work differently--it just disallows some non-conforming code--so should
be perfectly fine for the test suite under all VS2017 builds.

This commit also fixes one failure under non-permissive mode.

This fixes a bug introduced in b68959e8
when passing in a two-dimensional, but conformable, array as the value
for a compile-time Eigen vector (such as VectorXd or RowVectorXd).  The
commit switched to using numpy to copy into the eigen data, but this
broke the described case because numpy refuses to broadcast a (N,1)
into a (N).

This commit fixes it by squeezing the input array whenever the output
array is 1-dimensional, which will let the problematic case through.
(This shouldn't squeeze inappropriately as dimension compatibility is
already checked for conformability before getting to the copy code).

When using `method_adaptor` (usually implicitly via a `cl.def("f",
&D::f)`) a compilation failure results if `f` is actually a method of
an inaccessible base class made public via `using`, such as:

    class B { public: void f() {} };
    class D : private B { public: using B::f; };

pybind deduces `&D::f` as a `B` member function pointer.  Since the base
class is inaccessible, the cast in `method_adaptor` from a base class
member function pointer to derived class member function pointer isn't
valid, and a cast failure results.

This was sort of a regression in 2.2, which introduced `method_adaptor`
to do the expected thing when the base class *is* accessible.  It wasn't
actually something that *worked* in 2.1, though: you wouldn't get a
compile-time failure, but the method was not callable (because the `D *`
couldn't be cast to a `B *` because of the access restriction).  As a
result, you'd simply get a run-time failure if you ever tried to call
the function (this is what #855 fixed).

Thus the change in 2.2 essentially promoted a run-time failure to a
compile-time failure, so isn't really a regression.

This commit simply adds a `static_assert` with an accessible-base-class
check so that, rather than just a cryptic cast failure, you get
something more informative (along with a suggestion for a workaround).

The workaround is to use a lambda, e.g.:

    class Derived : private Base {
    public:
        using Base::f;
    };

    // In binding code:
    //cl.def("f", &Derived::f); // fails: &Derived::f is actually a base
                                // class member function pointer
    cl.def("f", [](Derived &self) { return self.f(); });

This is a bit of a nuissance (especially if there are a bunch of
arguments to forward), but I don't really see another solution.

Fixes #1124

This changes the caster to return a reference to a (new) local `CharT`
type caster member so that binding lvalue-reference char arguments
works (currently it results in a compilation failure).

Fixes #1116

* Upgrades to latest stable Xcode (9)

* Fixes build error in the OS X/Python 3 build.

Fixes #1117

This also matches the Eigen example for the row-major case.

This also enhances one of the tests to trigger a failure (and fixes it in the PR).  (This isn't really a flaw in pybind itself, but rather fixes wrong code in the test code and docs).

The entire test file is already marked as requiring numpy; it isn't
needed on the individual test.

`PyArray_DescrConverter_` doesn't steal a reference to the argument,
and so the passed arguments shouldn't be `.release()`d.

`type_descr` is now applied only to the final signature so that it only
marks the argument types, but not nested types (e.g. for tuples) or
return types.

MSCV does not allow `&typeid(T)` in constexpr contexts, but the string
part of the type signature can still be constexpr. In order to avoid
`typeid` as long as possible, `descr` is modified to collect type
information as template parameters instead of constexpr `typeid`.
The actual `std::type_info` pointers are only collected in the end,
as a `constexpr` (gcc/clang) or regular (MSVC) function call.

Not only does it significantly reduce binary size on MSVC, gcc/clang
benefit a little bit as well, since they can skip some intermediate
`std::type_info*` arrays.

The current C++14 constexpr signatures don't require relaxed constexpr,
but only `auto` return type deduction. To get around this in C++11,
the type caster's `name()` static member functions are turned into
`static constexpr auto` variables.

When Travis changes their default Python 3.x, it breaks any hardcoded
version selection. Fix: make pyenv activate everything (2.7, 3.x) and
use whichever Python 3.x is on by default.

[skip appveyor]

* Expand documentation to include explicit example of py::module::import 
  where one would expect it.

* Describe how to use unbound and bound methods to class Python classes.

[skip ci]

E.g. trying to convert a `list` to a `std::vector<int>` without
including <pybind11/stl.h> will now raise an error with a note that
suggests checking the headers.

The note is only appended if `std::` is found in the function
signature. This should only be the case when a header is missing.
E.g. when stl.h is included, the signature would contain `List[int]`
instead of `std::vector<int>` while using stl_bind.h would produce
something like `MyVector`. Similarly for `std::map`/`Dict`, `complex`,
`std::function`/`Callable`, etc.

There's a possibility for false positives, but it's pretty low.

Fixes #1069.

Fixes #1075.

`PyNumber_Float()` and `PyNumber_Long()` return new references.

To avoid an ODR violation in the test suite while testing
both `stl.h` and `std_bind.h` with `std::vector<bool>`,
the `py::bind_vector<std::vector<bool>>` test is moved to
the secondary module (which does not include `stl.h`).

[skip appveyor]