Support module-level skips

[cdleonard]

It seems that pytest.mark.skip only works on specific mypy tests.

• It's not possible to use pytestmark at the module level.
• It's not possible to use pytest.skip(allow_module_level=True) either

This means that if a test only works on newer versions of python each individual test needs to be marked for skipping. This is silly, for example if you do from typing import Literal then nothing will work on python 3.7 so it's better to skip the entire file in a single place.