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: Idc4d6259d4eb2db66e7122535bc42f8647049a06
Merge "Activate pep8 check that _ is imported"