Generics true-positive for newly created different generics

[davidhalter]

Pyright and mypy warn about all four expressions below (a-d):

from typing import Self

class Size[T]:
    def __init__(self, value: T) -> None:
        self.value = value

    def end(self) -> None:
        a = Size(1).__sub__(self)
        b = Size(1) - self
        c = self.__sub__(Size(1))
        d = self - Size(1)

    def __sub__(self, other: Self) -> 'Size':
        return self

zuban only warns about expressions c and d.

Originally posted by @correctmost in #304

[davidhalter]

I realized that the cause of this is that the Size type var variances are not calculated at the time when Size(1) is initialized. This is a bit tricky to fix, but it's definitely a bug. I'm in general not the biggest fan of TypeVar variance inference, but that's a different discussion.