Handle Self wrapped in Annotated when checking the expected self type

[EmmanuelNiyonshuti]

Fixes #21917

Self wrapped in Annotated was not being recognized as an expected self type. This PR updates is_expected_self_type to recurse through Annotated when checking unbound types.

[A5rocks]

I'm surprised we don't already simply trim out Annotated in all types! But this makes sense to me otherwise.

[A5rocks]

Could you add a check for a classmethod too? I think you missed that case.

[EmmanuelNiyonshuti]

I missed that, thanks.

[github-actions]

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

[EmmanuelNiyonshuti]

Out of curiosity(not entirely sure if it's actually an issue), while snooping around to fix this issue, I noticed mypy is okay with this:

class Foo:
      def fails[T: Self](self: Annotated[T, "tag"], x: int) -> None: pass

I might be missing a lot of things since I'm fairly new to type checking semantics, but I think I couldn't find anything about this in PEP 673, and no other type checker allows this(I tried pyright, zuban, and ty). Is this intended on mypy's side?

[Moussa-Mikhail]

I've also found that the following has the same issue:

from typing import Self

type Test[T] = T

class Foo:
      # error: Method cannot have explicit self annotation and Self type  [misc]
      def also_fails(self: Test[Self], x: int) -> None: pass

Not a practical issue but it does seem odd.