Generate an error if trying to use @final with TypedDict #7849

@final doesn't work as expected with TypedDicts, but mypy doesn't complain about it:

from typing_extensions import final, TypedDict

@final  # No error here
class C(TypedDict):
    x: float
    y: int
    z: str

class D(C):  # No error here
    pass

This is confusing, since users would expect that @final prevents structural subtyping, but that's not the case. The simplest fix would be to generate an error if @final is used in a TypedDict definition.

Hints: This should be a pretty simple thing to fix. Look at what happens in analyze_class in mypy/semanal.py.

Originally reported in #7845.