Enable some off-by-default checks

Some of 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: I6c880ba9fcde60cbebdea4aa459b42c6b92d181c

tox.ini

@@ -58,6 +58,9 @@ commands = oslo_debug_helper {posargs}
 [flake8]
 show-source = True
 builtins = _
+# H106: Don’t put vim configuration in source files
+# H203: Use assertIs(Not)None to check for None
+enable-extensions=H106,H203
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [hacking]