[3.9] bpo-42195: Ensure consistency of Callable's __args__ in collections.abc and typing (GH-23060)

[Fidget-Spinner]

This patch fixes bpo-42195 via changing the GenericAlias builtin and subclassing for collections.abc.Callable's __class_getitem__. The changes are:

1. Made GenericAlias subclassable.
2. Subclassed it in collections.abc.Callable's __class_getitem__ to allow for consistentency with typing.Callable's __args__. This includes flattening the __args__.

>>> import collections.abc
>>> alias = collections.abc.Callable[[int, str], dict]

>>> alias
collections.abc.Callable[[int, str], dict]

>>> alias.__args__
(int, str, dict)

3. Added tests for the above.
4. Loosened type checks for Callable's argtypes in Callable[[argtypes], resulttype] to prepare for PEP 612.

https://bugs.python.org/issue42195

(cherry picked from commit 463c7d3)

[gvanrossum]

Thanks! Just what I had in mind.

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[gvanrossum]

I don't understand why you get test failures for test_ttk_guionly, but I don't think it's your fault. Possibly it'll pass now the tests are bein rerun.