fixes #2191

If the default argument value is a class, and not an instance of a
class, `a.value.attr("__repr__")` raises a `ValueError`. Switching to
`repr(a.value)` makes this use case work.

Fixes #2028

Adds requirements file too.

Fix https://github.com/pybind/pybind11/issues/2318

Tested with 2.7.18rc1, built with Py_UNICODE_SIZE 4.
Change also tested with Python 3.8.

- look for windows python lib when using mingw & msys
  - if not found, then look for system python lib as before

* Fix undefined memoryview format

* Add missing <algorithm> header

* Add workaround for py27 array compatibility

* Workaround py27 memoryview behavior

* Fix memoryview constructor from buffer_info

* Workaround PyMemoryView_FromMemory availability in py27

* Fix up memoryview tests

* Update memoryview test from buffer to check signedness

* Use static factory method to create memoryview

* Remove ndim arg from memoryview::frombuffer and add tests

* Allow ndim=0 memoryview and documentation fixup

* Use void* to align to frombuffer method signature

* Add const variants of frombuffer and frommemory

* Add memory view section in doc

* Fix docs

* Add test for null buffer

* Workaround py27 nullptr behavior in test

* Rename frombuffer to from_buffer

- `PyArray_NewFromDescr_` should have been using `Py_intptr_t const *`
  - https://github.com/numpy/numpy/blob/18a6e3e505ee416ddfc617f3e9afdff5a031c2c2/numpy/core/src/multiarray/ctors.h#L5-L8
- `PyArray_GetArrayParamsFromObject_` should be using `NPY_BOOL`
  - https://github.com/numpy/numpy/blob/18a6e3e505ee416ddfc617f3e9afdff5a031c2c2/numpy/core/src/multiarray/ctors.c#L1350-L1355
  - https://docs.scipy.org/doc/numpy-1.13.0/reference/c-api.dtype.html#c.npy_bool
- `PyArray_DescrNewFromType` is at offset 96
  - https://github.com/numpy/numpy/blob/36e017194c32a53cf2965a513243cb3c348095df/numpy/core/code_generators/numpy_api.py#L141
- `array_t` constructor that takes a size and a pointer should take `ssize_t`.
  - Fixes #1599

This avoids a potential conflict with names in the same scope of the
same name as the embedded module, like namespaces or other global
variables.

Fixes #2172

* Change NAMESPACE_BEGIN and NAMESPACE_END macros into PYBIND11_NAMESPACE_BEGIN and PYBIND11_NAMESPACE_END

* Fix sudden HomeBrew 'python not installed' error

* Sweep difference in 'Class.__init__() must be called when overriding __init__' error message between CPython and PyPy under the rug

* Homebrew updated to 3.8 yesterday.

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

* Error out eval_file

* Enable dynamic attribute support for Pypy >= 6

* Add a test for dynamic attribute support

* Skip test for eval_file on pypy

* Workaround for __qualname__ on PyPy3

* Add a PyPy3.6 7.3.0 build

* Only disable in PyPy3

* Fix travis testing

* No numpy and scipy for pypy

* Enable test on pypy2

* Fix logic in eval_file

* Skip a few tests due to bugs in PyPy

* scipy wheels are broken. make pypy2 a failrue

Co-authored-by: Andreas Kloeckner <inform@tiker.net>