Incorrect bound method signature involving `Self`

[ibraheemdev]

Summary

from typing import Self, reveal_type

class Foo[T]:
    def foo(self) -> T:
        raise NotImplementedError

class Bar:
    def _(self, x: Foo[Self]):
        reveal_type(x.foo)  # revealed: bound method Foo[Self@_].foo() -> Foo[Self@_]

def f[T: Bar](x: Foo[T]):
    reveal_type(x.foo)  # revealed: bound method Foo[T@f].foo() -> T@f

The return type of the first is incorrect.

Originally found in astral-sh/ruff#21685 (comment). I bisected this to astral-sh/ruff#20677.