Update `TypedDict` to match updated PEP-728

[mharding-hpe]

typing_extensions 4.10 added support for PEP-728. However, the PEP was updated last month to change how it is being implemented. Instead of the __extra_items__ class attribute, it now is using the extra_items class parameter. It would be good to make the corresponding change in typing_extensions, to match the updated PEP-728.

[JelleZijlstra]

I started working on this but I'm not sure I'm happy with the runtime behavior proposed in the current version of the PEP, and there was some more talk on Discuss about possible alternative spellings. I'll look at it again when I find some time.

[mharding-hpe]

Thank you -- much appreciated. I am eagerly awaiting this new feature, in whatever form it finally arrives. :)

[chuckwondo]

I just stumbled upon PEP 728 and I'd like to see the updated proposal implemented as well. Any further thoughts here?

This is also holding up mypy support: python/mypy#18176