`isinstance` doesn't narrow when using union

[DetachHead]

a: int | str
assert isinstance(a, int | None)
reveal_type(a)

playground

[JelleZijlstra]

This seems specific to | None. If you do assert isinstance(a, int | list) instead, it correctly narrows to int.

[KotlinIsland]

@SakshiSingh25 Start here: https://github.com/python/mypy/blob/master/CONTRIBUTING.md

This issue is likely due to strict_optional in some way, so search for that.