bpo-43795: Add a test for Stable ABI symbol availability using ctypes (GH-26354)

This is a cross-platform check that the symbols are actually
exported in the ABI, not e.g. hidden in a macro.

Caveat: PyModule_Create2 & PyModule_FromDefAndSpec2 are skipped.

These aren't exported on some of our buildbots. This is a bug
(bpo-44133). This test now makes sure all the others don't regress.