Move error messages for keyword arguments with ParamSpec #21001

Feature

At the moment, you need --extra-checks to catch this pedantic thing:

from typing import Concatenate, ParamSpec
from collections.abc import Callable

P = ParamSpec("P")

def f(a: Callable[P, None]) -> Callable[Concatenate[int, P], None]:
    def inner(x: int, *args: P.args, **kwargs: P.kwargs) -> None:  # proposal: warn here
        a(*args, **kwargs)
    
    return inner  # warns on this

Instead, @dhruvmanila came up with a different rule: warn on the definition of functions that have a keyword argument + ParamSpec *args/**kwargs. I think this is better, because we're trying to move away from flags which modify behavior strangely AFAIK. Then, regardless of error, we would treat x as positional-only.

Pitch

We would remove any custom subtyping (iirc, that's how --extra-checks works now), instead just replacing functions with a better version if it has *args: P.args, **kwargs: P.kwargs. Then we would have a disabled-by-default error code for this new check.