Add flake8-bugbear and flake8-warnings

These catch issues that aren't found by any of the other tools available.

Included changes:

• mypy now allows a way to disable a specific warning on a line, so remove the workaround in mycnf.Cnf.init. This required changing the param in _normalize_keys, to make mypy happy.
• Fix a missing assert in test_mycnf.py that is found by bugbear.
• Change a setattr in test_mycnf.py to make bugbear happier, too.