Activate pep8 check that _ is imported

Currently translatable messages are not used, so there are no uses
of _.

This will ensure if _ is used in the future pep8 won't assume
it is provided as a builtin, and to be consistent with the rest of
oslo.

Change-Id: I5b840eaf1a55d3df40ed56af19ea7349f167b5a9

tox.ini

@@ -26,7 +26,6 @@ commands = python setup.py build_sphinx
 [flake8]
 show-source = True
 exclude = .tox,dist,doc,*.egg,build
-builtins = _
 
 [hacking]
 import_exceptions =