Better error messages for some TypeVar init cases

[sobolevn]

While working on #11379 I found several error message that I don't quite understand:

A = TypeVar('A', covariant=False, contravariant=True)
# E: TypeVar 'covariant' may only be 'True'

B = TypeVar('B', covariant=True, contravariant=False)
# E: TypeVar 'contravariant' may only be 'True'

But:

A = TypeVar('A', covariant=True, contravariant=True)
# E: TypeVar cannot be both covariant and contravariant

B = TypeVar('B', covariant=True, contravariant=True)
# E: TypeVar cannot be both covariant and contravariant

[sobolevn]

Btw, this message is not tested.

[sobolevn]

I think that I am going to change the logic a bit as well. Because this is valid python:

>>> from typing import TypeVar
>>> A = TypeVar('A', contravariant=False)

No need to forbid that 👍

[sobolevn]

Thanks for the merge 😊