Narrow for frozendict membership check

[hauntsaninja]

def narrow(key: int | None, mapping: frozendict[int, str]) -> None:
    if key in mapping:
        reveal_type(key)  # N: Revealed type is "builtins.int"

Linking #21447

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅