[3.15] gh-155485: Skip test_pyexpat.test_subparser_inherits_reparse_deferral when Expat < 2.6.0 (GH-155554)

[3.15] gh-155485: Skip test_pyexpat.test_subparser_inherits_reparse_deferral when Expat < 2.6.0 (GH-155554) #128791