fix: include PYTHON_IS_DEBUG

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