Remove useless flake8 config + test support code (#2221)

We've depended on Click 7.x ever since we broke CI systems across the
world (oops lol) and flake8-mypy was purged a fair bit back: #1867

Also remove the primer tests import in tests/test_black.py because it's
annoying when just trying to actually target tests/test_black.py tests.
`pytest -k test_black.py` doesn't do what you expect due to that import.