* gh-134632: Fix `build-details.json` to use `INCLUDEPY` path
Fix ``build-details.json`` generation to use ``INCLUDEPY``, in order to
reference the ``pythonX.Y`` subdirectory of the include directory, as
required in :pep:`739`, instead of the top-level include directory.
* test_build_details: Add tests for the c_api section
* test_build_details: Expect pkgconfig for CPython unconditionally
Fix `build-details.json` generation to use the correct `c_api.headers`
key as defined in PEP 739, instead of `c_api.include`.
Co-authored-by: Filipe Laíns 🇵🇸 <lains@riseup.net>
