Implement PEP 692

@franekmagiera

PEP-692 was accepted, but without the syntax change. That means there's very little to do in CPython, but a few changes should be made:

https://docs.python.org/3.12/library/typing.html#typing.Unpack should mention this new use for Unpack
https://docs.python.org/3.12/whatsnew/3.12.html should mention the PEP
Maybe some aspects of typing.py assume Unpack is only used for PEP 646 and should be updated

And maybe something else I'm forgetting.

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