Improve ABI/feature selection, add new header for it.
Add a test that Python headers themselves don't use
Py_GIL_DISABLED in abi3t: abi3 and abi3t ought to be the
same except the _Py_OPAQUE_PYOBJECT differences.
This is done using the GCC-only poison pragma.
Co-authored-by: Victor Stinner <vstinner@python.org>
