Backport generic NamedTuple and TypedDict support

[JelleZijlstra]

Python 3.11 will support generic NamedTuples (python/cpython#92027) and TypedDicts (python/cpython#27663) at runtime. If there is community interest, we should backport support for these features into typing-extensions. I'm not planning to work on this myself but I'll accept patches.

[AlexWaygood]

I'd be interested in working on this if nobody else wants to, but I've got quite a lot on for the next two weeks or so, so I'm happy to let somebody else tackle it if they can do it more promptly!

[Bluenix2]

As discovered in Python Discord's #type-hinting channel (09/05/2022), typing_extensions 4.2 release appears to support this already.

That said, it appears to still be missing for Python 3.7 and 3.8. This CI run of mine is on Python 3.7 and locally I can reproduce it on my Python 3.8.10 installaton (py -3.8-64 -c "import discord_typings"):

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\%username%\Projects\discord-typings\discord_typings\__init__.py", line 5, in <module> 
    from .gateway import *
  File "C:\Users\%username%\Projects\discord-typings\discord_typings\gateway.py", line 267, in <module>
    ReadyEvent = GenericDispatchEvent[Literal['READY'], ReadyData]
TypeError: '_TypedDictMeta' object is not subscriptable

[JelleZijlstra]

The current support is accidental, not tested, and doesn't cover all supported Python versions, so I don't think it really makes a difference.

[Bluenix2]

It gives hints as to what one could do to make this pass on all versions ☝️

[julien-blanchon]

I love this ! I hope TypedDict will also be include soon. Thanks you @JelleZijlstra @AlexWaygood