fix detection of whether MemberExpr.expr represents self/cls in…

[bzoracler]

…side methods

Fixes #20479.

[bzoracler]

.mod.var was revealed to be Any in the body of __new__() and pos_only_self() before this PR. See e.g. mypy Playground for a reproducer:

class A:
    import math

    def __new__(cls) -> "A":
        reveal_type(cls.math.pi)  # N: Revealed type is "Any"
        return cls()

    def pos_or_named_self(self) -> None:
        reveal_type(self.math.pi)  # N: Revealed type is "builtins.float"

    def pos_only_self(self, /) -> None:
        reveal_type(self.math.pi)  # N: Revealed type is "Any"

    @classmethod
    def clsmethod(cls) -> None:
        reveal_type(cls.math.pi)  # N: Revealed type is "builtins.float"

[github-actions]

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