Reject more covariant overrides

[hauntsaninja]

Follow up to #3208. We should also reject things like:

from typing import TypeVar, Generic
T = TypeVar("T", covariant=True)
class Foo(Generic[T]):
    def __init__(self, value: T) -> None:
        self.foo = value

[ilevkivskyi]

Is this a duplicate of #735? In particular #735 (comment)