ExceptionGroup should be generic #91549

Thanks to PEP 654, 3.11 will have the new except* syntax. Let's take this sample:

try:
    do_failing_things()
except* ValueError as eg:
    reveal_type(eg)

In the except* block, eg is an ExceptionGroup instance that in its exceptions attribute contains either ValueError instances or more nested ExceptionGroups containing ValueErrors. I expect that many except* blocks will do operations specific to the exception type they operate on, but right now there's no way to represent that information in the type system.

To fix this, we can make ExceptionGroup (and BaseExceptionGroup) generic. In the example, eg would be of type ExceptionGroup[ValueError] (to static type checkers), indicating that all the contained exceptions are ValueErrors.

Recommendations:

• In CPython, add the C equivalent of __class_getitem__ = GenericAlias to ExceptionGroup and BaseExceptionGroup.
• In typeshed, make ExceptionGroup (and base) generic over a TypeVar("ExceptionT", bound=Exception), and type its exceptions attribute as tuple[ExceptionT | ExceptionGroup[ExceptionT], ...].
• In type checkers, for code of the form except* T as eg:, infer eg to be of type ExceptionGroup[T]. (Or BaseExceptionGroup[T] if T does not inherit from Exception.)

If we're in agreement, I can implement the first two recommendations.

Links:

• https://peps.python.org/pep-0654/#except (spec)
• https://github.com/python/typeshed/blob/7f9be7fd00ff86750a3d983681a9ef94b5511e2c/stdlib/builtins.pyi#L1752 (current typeshed stubs)

cc @iritkatriel @gvanrossum for PEP 654, @srittau @AlexWaygood for typeshed, @erictraut as a type checker maintainer.