Enable some off-by-default checks

Some of the available checks are disabled by default, like:
[H106] Don’t put vim configuration in source files
[H203] Use assertIs(Not)None to check for None

Change-Id: I369cff1c0f7f3cd3f5bcf3785b6904c9326c6759

tox.ini

@@ -49,6 +49,7 @@ commands =
 show-source=True
 ignore= H105,E123,E226,N320
 builtins= _
+enable-extensions = H106,H203
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,*sqlalchemy/alembic/versions/*,demo/,releasenotes
 
 [testenv:wheel]