False positive error on `**kwargs` with `Mapping[Literal, Any]` type

https://mypy-play.net/?mypy=master&python=3.12&flags=verbose&gist=c3db401cf676c8c593dec1c525f7e748

Mypy Version: 1.20.0+dev.d06d3d9cfd6611c0e64c0df59fc0449754e47ed8

Fixed by: #20419

See Also: #20416

from typing import Literal, Mapping, Iterable
def func(a: int, b: int) -> None: ...

class GOOD_KW:
    def keys(self) -> Iterable[Literal["a", "b"]]: ...
    def __getitem__(self, key: str) -> int: ...

class BAD_KW:
    def keys(self) -> Iterable[Literal["one", 1]]: ...
    def __getitem__(self, key: str) -> int: ...

def test(
    good_kw: GOOD_KW,
    bad_kw: BAD_KW,
    good_dict: dict[Literal["a", "b"], int],
    bad_dict: dict[Literal["one", 1], int],
    good_mapping: Mapping[Literal["a", "b"], int],
    bad_mapping: Mapping[Literal["one", 1], int],
) -> None:
    func(**good_kw)
    func(**bad_kw)  # E: Argument after ** must have string keys
    func(**good_dict)
    func(**bad_dict)  # E: Argument after ** must have string keys
    func(**good_mapping)  # ❌️ False positive
    func(**bad_mapping)  # E: Argument after ** must have string keys

Start with the linked mypy-play reproducer and compare the GOOD_KW, BAD_KW, dict, and Mapping cases. Read #20419, which the issue identifies as the fix, and #20416 for related context. Done means the Mapping case no longer reports a false positive while the invalid non-string key cases still produce errors.