[3.12] gh-109230: test_pyexpat no longer depends on the current directory (GH-109233...

[3.12] gh-109230: test_pyexpat no longer depends on the current directory (GH-109233) (#109241)

gh-109230: test_pyexpat no longer depends on the current directory (GH-109233)

Fix test_pyexpat.test_exception(): it can now be run from a directory
different than Python source code directory. Before, the test failed
in this case.

Skip the test if Modules/pyexpat.c source is not available. Skip also
the test on Python implementations other than CPython.
(cherry picked from commit e55aab95786e0e9fb36a9a1122d2d0fb3d2403cd)

Co-authored-by: Victor Stinner <vstinner@python.org>