Emit [mutable-override] for covariant override of attribute with method

[brianschubert]

Fixes #18052

Given:

# flags: --enable-error-code=mutable-override
from typing import Callable

class Parent:
    func: Callable[[str], None]

class Child(Parent):
    def func(self, x: object) -> None: pass 

Before:

Success: no issues found in 1 source file

After:

main.py:7 error: Covariant override of a mutable attribute (base class "Parent" defined the type as "Callable[[str], None]", override has type "Callable[[object], None]")
Found 1 error in 1 file (checked 1 source file)

[github-actions]

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

[hauntsaninja]

Thank you!