`nullptr` is not expected to work in this case.

gcc 7 is now in debian testing ("buster"), with a proper stable upstream
release; this updates the associated travis-ci to use "buster" (rather
than "sid"), and removes the build from allow_failures.

[skip ci]

This fixes #856. Instead of the weakref trick, the internals structure
holds an unordered_map from PyObject* to a vector of references. To
avoid the cost of the unordered_map lookup for objects that don't have
any keep_alive patients, a flag is added to each instance to indicate
whether there is anything to do.

Using `std::type_info::operator==` fails under libc++ because the .so
is loaded with RTLD_LOCAL.  libc++ considers types under such .sos
distinct, and so comparing typeid() values directly isn't going to work.

This adds a custom hasher and equality class for the type lookup maps
when not under stdlibc++, and adds a `detail::same_type` function to
perform the equality test.  It also converts a few pointer arguments to
const lvalue references, particularly since doing the pointer
comparison wasn't technically valid to being with (though in practice,
appeared to work everywhere).

This fixes #912.

Fixes a race condition when multiple threads try to acquire the GIL
before `detail::internals` have been initialized. `gil_scoped_release`
is now tasked with initializing `internals` (guaranteed single-threaded)
to ensure the safety of subsequent `acquire` calls from multiple threads.

Wrapped long lines and removed a few trailing spaces.

Fixes the issue as described in the comments of commit e27ea47c. This
just adds `enable_if_t<std::is_move_constructible<T>::value>` to
`make_move_constructor`. The change fixes MSVC and is harmless with
other compilers.

CLion slows to a crawl when evaluating the intricate `PYBIND11_NUMPY_DTYPE`
macro. This commit replaces the macro cascade with a simple `(void)0`
to ease IDE evaluation.

Debian stretch was just released, so `debian:testing` and
`debian:stetch` are starting to diverge; this commit keeps the travis-ci
docker image on stretch for gcc6 and clang3.9.

Debian has also moved gcc 7 from experimental to unstable, so this
switches the gcc7 build to `sid`.  Once it migrates to `testing` I'll
switch the gcc 7 build docker image to `testing` and take it out of
failure-allowed.

This allows named capsules to be constructed with `py::capsule`.

./tools/check-style.sh fails on stock OS X currently; this fixes it:

- use pipes directly rather than exec redirection (macOS's ancient
  version of bash fails with the latter)
- macOS's ancient bash doesn't support '\e' escapes in `echo -e`;
  replace with \033 instead
- BSD grep doesn't support GREP_COLORS, but does allow GREP_COLOR.
  Adding both doesn't hurt GNU grep: GREP_COLOR is deprecated, and won't
  be used when GREP_COLORS is set.
- BSD grep doesn't collapse multiple /'s in the listed filename, so
  failures under `include/` would should up as
  `include//pybind11/whatever.h`.  This removes the / from the include
  directory argument.

Minor other changes:
- The CRLF detection runs with -l, so GREP_COLORS wasn't doing
  anything; removed it.
- The trailing whitespace test would trigger on CRLFs, but the CR would
  result in messed up output.  Changed the test to just match trailing
  spaces and tabs, rather than all whitespace.

The clang 4.0/cpp17 build wasn't enabling -flto because the system
linker didn't like the output generated by clang for some reason.  This
switches the build to use llvm's lld instead, which lets -flto work
again (and links considerably faster, too).

numpy 1.13.0 fails with pypy 5.7.1, so this upgrades to 5.8.0.  I've
also uploaded pre-built .whl files to imaginary.ca (checked every 4
hours and rebuilt if needed), and list that as an extra pypi location
under the pypy pip install to avoid the long travis pypy build times for
a new release or branch.

This commit allows multiple inheritance of pybind11 classes from
Python, e.g.

    class MyType(Base1, Base2):
        def __init__(self):
            Base1.__init__(self)
            Base2.__init__(self)

where Base1 and Base2 are pybind11-exported classes.

This requires collapsing the various builtin base objects
(pybind11_object_56, ...) introduced in 2.1 into a single
pybind11_object of a fixed size; this fixed size object allocates enough
space to contain either a simple object (one base class & small* holder
instance), or a pointer to a new allocation that can contain an
arbitrary number of base classes and holders, with holder size
unrestricted.

* "small" here means having a sizeof() of at most 2 pointers, which is
enough to fit unique_ptr (sizeof is 1 ptr) and shared_ptr (sizeof is 2
ptrs).

To minimize the performance impact, this repurposes
`internals::registered_types_py` to store a vector of pybind-registered
base types.  For direct-use pybind types (e.g. the `PyA` for a C++ `A`)
this is simply storing the same thing as before, but now in a vector;
for Python-side inherited types, the map lets us avoid having to do a
base class traversal as long as we've seen the class before.  The
change to vector is needed for multiple inheritance: Python types
inheriting from multiple registered bases have one entry per base.

Fixes #896.

From Python docs: "Once an iterator’s `__next__()` method raises
`StopIteration`, it must continue to do so on subsequent calls.
Implementations that do not obey this property are deemed broken."

pytest raises an error if it recurses into these directories.

Passing utf8 encoded strings from python to a C++ function taking a
std::string was broken.  The previous version was trying to call
'PyUnicode_FromObject' on this data, which failed to convert the string
to unicode with the default ascii codec. Also this incurs an unnecessary
conversion to unicode for data this is immediately converted back to
utf8.

Fix by treating python 2 strings the same python 3 bytes objects, and just
copying over the data if possible.

libc++ 3.8 (and possibly others--including the derived version on OS X),
doesn't define the macro, but does support std::experimental::optional.
This removes the extra macro check and just assumes the header existing
is enough, which is what we do for <optional> and <variant>.

MSVC 2015 Update 3 and 2017 can handle enough expression SFINAE
to make this work now.

Fix spelling

Py_Finalize could potentially invoke code that calls `get_internals()`,
which could create a new internals object if one didn't exist.
`finalize_interpreter()` didn't catch this because it only used the
pre-finalize interpreter pointer status; if this happens, it results in
the internals pointer not being properly destroyed with the interpreter,
which leaks, and also causes a `get_internals()` under a future
interpreter to return an internals object that is wrong in various ways.

[skip ci]

Fixes #887.

`accessor` currently relies on an implicit default copy constructor, but that is deprecated in C++11 when a copy assignment operator is present and can, in some cases, raise deprecation warnings (see #888).  This commit explicitly specifies the default copy constructor and also adds a default move constructor.

[skip ci]

This reimplements the std::reference_wrapper<T> caster to be a shell
around the underlying T caster (rather than assuming T is a generic
type), which lets it work for things like `std::reference_wrapper<int>`
or anything else custom type caster with a lvalue cast operator.

This also makes it properly fail when None is provided, just as an
ordinary lvalue reference argument would similarly fail.

This also adds a static assert to test that T has an appropriate type
caster.  It triggers for casters like `std::pair`, which have
return-by-value cast operators.  (In theory this could be supported by
storing a local temporary for such types, but that's beyond the scope
of this PR).

This also replaces `automatic` or `take_ownership` return value policies
with `automatic_reference` as taking ownership of a reference inside a
reference_wrapper is not valid.

test_issues is deprecated, and the following commit adds other, related
reference_wrapper tests.

The new version of pytest now reports Python warnings by default. This
commit filters out some third-party extension warnings which are not
useful for pybind11 tests.

This commit also adds `doc()` to `object_api` as a shortcut for the
`attr("__doc__")` accessor.

The module macro changes from:
```c++
PYBIND11_PLUGIN(example) {
    pybind11::module m("example", "pybind11 example plugin");
    m.def("add", [](int a, int b) { return a + b; });
    return m.ptr();
}
```

to:

```c++
PYBIND11_MODULE(example, m) {
    m.doc() = "pybind11 example plugin";
    m.def("add", [](int a, int b) { return a + b; });
}
```

Using the old macro results in a deprecation warning. The warning
actually points to the `pybind11_init` function (since attributes
don't bind to macros), but the message should be quite clear:
"PYBIND11_PLUGIN is deprecated, use PYBIND11_MODULE".