Activate pep8 check that _ is imported
Activate pep8 check that _ is imported

Remove the specification in tox.ini that _ is a builtin so that
it will no longer assume that _ does not need to be imported.
This helps ensure that the _ from i18n is used.

Activating this check did not flag any violations.

Change-Id: I9cae8a2e16ff49b8272f42925228f1524ddf0fea

openstack/mox3