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: I7b7f96cbece2192432a9b7b6a4355ebf0c868926
Merge "Activate pep8 check that _ is imported"