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: Ia8c7efe5bb30294c349b504d569e6ad7c602f246

EntropyWorks/bifrost,devananda/bifrost,openstack/bifrost,openstack/bifrost,bcornec/bifrost,bcornec/bifrost,devananda/bifrost,EntropyWorks/bifrost