Disable `assignment-from-none` for methods

[cdce8p]

# pylint: disable=no-self-use

class Parent:
    def func(self) -> int | None:
        return None

    def serialize(self) -> str:
        x = self.func()  # assignment-from-none
        return f"Result: {x if x else ''}"

class Child(Parent):
    def func(self) -> int:
        return 42

In the example above, assignment-from-none is emitted on a method call even though func() could be overwritten and not return None.

Proposal
Limit the assignment-from-none check to functions. Methods should only be considered if marked with @typing.final. It might also work to check the return type itself to see if it's only -> None.

Reason
We can't reliably determine if a method is ever overwritten and thus return something else then None.