Check covariant variable used in an invariant position in a return type

[JukkaL]

Covariant type variable should not be used in the return type in an invariant position, for example like this:

CV = TypeVar('CV', covariant=True)

class A(Generic[CV]):
    def foo(self) -> List[CV]: ...   # Error, should be 
        # Sequence[CV] or similar, since List is invariant

There is probably a similar rule for contravariance.