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bbfcf6e1 - [ty] `__class_getitem__` is a classmethod (#20192)

[ty] `__class_getitem__` is a classmethod (#20192) ## Summary `__class_getitem__` is [implicitly a classmethod](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__). ## Test Plan Added regression test.