* add uninstall target for cmake

* only add target when built as master project

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

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

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