For 'get_python_inc', bypass missing config include dir

[jaraco]

• Add xfail test capturing missed expectation. Ref get_python_inc returns wrong include directory for bundled Pythons #178.
• In _get_python_inc_posix, only return an extant path from the sysconfig. Fixes get_python_inc returns wrong include directory for bundled Pythons #178.

[mattip]

What would your feeling be about adding pypy to the CI matrix? On the one hand it would be good to test everything, on the other PyPy is rather niche, things may break, and it could slow down development. The python action accepts PyPy versions like 'pypy3.9'

[jaraco]

I'm not opposed, and it would probably be worthwhile to have at least one PyPy in the matrix.