`TypedDict` should extend `dict` not `Mapping`

[KotlinIsland]

And it should override the clear method to be Never.

[DetachHead]

the only way to do this without breaking the functionality that bans clear from being called on TypedDicts would be to override its clear method with clear: Never, but that wouldn't work because pyright doesn't complain when you use objects typed as Never

so this change relies on #97

[DetachHead]

this would also make reportInvalidCast less annoying (see here)

[DetachHead]

eh, i don't think we should do this, it becomes less safe even if we make clear typed as Never:

foo: dict[{"a": int}]

bar: dict[str, int] = foo

bar.clear() # no error

[KotlinIsland]

Is this only because we don't get an error about unreachable?

[DetachHead]

no it's because the Never is widened to something that's not Never