The main point is to remove one not particularly useful customization
point from setup.cfg, and the associated logic in setupext.py.

Minor additional changes in _tkagg.cpp: use METH_VARARGS instead of the
opaque "1" (one can check in Python.h that the values indeed match),
and just use #ifdef _WIN32 to check for Windows-ness
(src/file_compat.h also uses that check so it works).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way