Reject bound covariant type variable in List argument

[LiuYuHui]

Fixes #734

[sobolevn]

What about Dict and (possibly) other types?

from typing import Dict, TypeVar

T_co = TypeVar('T_co', covariant=True)
T_contra = TypeVar('T_contra', contravariant=True)

def cov(a: Dict[T_co, T_co]) -> Dict[T_co, T_co]:
    ...

def contra(a: Dict[T_contra, T_contra]) -> Dict[T_contra, T_contra]:
    ...

reveal_type(cov({'a': 1}))
reveal_type(contra({'a': 1}))

Outputs:

ex.py:12: error: Cannot infer type argument 1 of "cov"
ex.py:12: note: Revealed type is 'builtins.dict[Any, Any]'
ex.py:13: error: Cannot infer type argument 1 of "contra"
ex.py:13: note: Revealed type is 'builtins.dict[Any, Any]'

[msullivan]

I don't think we want to take a PR that does this only for List, and not other things. Happy to discuss how to generalize it.

(I'm closing the PR as part of trying to tidy up, but I'm happy to work with you on moving forward if you're still interested)