bpo-45359: Support TopologicalSorter type subscript

[JacobHayes]

Add TopologicalSorter.__class_getitem__ = classmethod(GenericAlias) to support runtime subscription.

https://bugs.python.org/issue45359

Automerge-Triggered-By: GH:isidentical

[the-knights-who-say-ni]

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[JacobHayes]

FYI: CLA has been signed, pending acceptance.

[isidentical]

This LGTM. The only missing thing is a test at https://github.com/python/cpython/blob/main/Lib/test/test_genericalias.py

[JacobHayes]

I think this is ready for another look when folks have time. Thanks!