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: I61ce619eeb37dc05f81015d37afc78a32838448a

tox.ini

@@ -245,6 +245,7 @@ commands = bindep test
 
 [flake8]
 show-source = True
+enable-extensions = H203,H106
 exclude=.eggs,.git,.tox,doc
 
 [hacking]