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: I34cf8b6358f59583cc178c029ebdd024f583dafe
Merge "Activate pep8 check that _ is imported"