Fix Any inference when unpacking iterators that don't directly inherit from typing.Iterator

[AlexWaygood]

Fixes #14819.

Mypy currently silently infers an Any type when unpacking an iterator that doesn't explicitly inherit from typing.Iterator (i.e., an iterator that's a structural subtype of typing.Iterator, but not a nominal subtype):

from typing import TypeVar

T = TypeVar("T")

class Foo:
    count: int
    def __init__(self) -> None:
        self.count = 0
    def __iter__(self: T) -> T:
        return self
    def __next__(self) -> int:
        self.count += 1
        if self.count > 3:
            raise StopIteration
        return self.count

a, b, c = Foo()
reveal_type(a)  # note: Revealed type is "Any"

However, we have enough information here to infer that the type of a should really be int. This PR fixes that bug.

There's discussion on the issue thread about an alternative solution that would involve changing some mypy behaviour that's been established for around 10 years. For now, I haven't gone for that solution.

[sobolevn]

Nice to see this ad-hoc code removed!

[sobolevn]

The other option here is to infer Any, because of def __iter__(self): pass

[AlexWaygood]

Yup, but as discussed in the issue, two long-standing tests fail if I make that change, so I'd rather not make that behaviour change in this PR 👍

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

Thanks!