Improve Liskov checks with overloads in subclass

[hauntsaninja]

The following program is type safe:

from typing import overload

class A: ...
class B: ...

class Parent:
    def dst(self, x: A | B) -> int | None: ...

class Child(Parent):
    @overload
    def dst(self, x: A) -> int: ...
    @overload
    def dst(self, x: B) -> None: ...

This came up in python/typeshed#9862