Reimplement TypedDict in a similar way to Python

[JelleZijlstra]

No description provided.

[JelleZijlstra]

Seems like TypedDict is implemented somewhat differently on CPython and in typing-extensions: on CPython it's a function, here it's a class and an instance of _TypedDictMeta. Maybe we should follow the CPython version more, which would mean concretely that is_typeddict(TypedDict) would be false and typing_extensions.TypedDict would no longer be a type.

[AlexWaygood]

Conceptually, it's better for TypedDict to be a function rather than a class, because it's a factory for creating new types, rather than a factory for creating instances of a specific type. I believe that's why the change was made over at CPython to change NamedTuple and TypedDict so that they're functions rather than classes.

[JelleZijlstra]

TypedDict now closely follows the implementation and tests in Python main, except for some interesting workarounds to get generics to work, and a few other details that don't work in old versions.

[AlexWaygood]

Looks good! Haven't reviewed the tests in depth, but I trust you're testing everything that's tested in CPython?

Looks like a few tests are failing on py312 btw

[JelleZijlstra]

All green now!

[AlexWaygood]

Looks great!