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: I2809ab2cce76dd31a04d3cca0e97cdad26c14cb4

tox.ini

@@ -50,6 +50,7 @@ commands = python setup.py testr --slowest --testr-args='--concurrency=1 {posarg
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.
 show-source = True
+enable-extensions = H203,H106
 ignore = E123,E125
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build