Activate pep8 check that _ is imported

Currently translatable messages are not used, so there are no uses
of _.

This will ensure if _ is used in the future pep8 won't assume
it is provided as a builtin.

Change-Id: Id70f5447a8035b76ab69844cf716abd17e232091
Merge "Activate pep8 check that _ is imported"
Updated from global requirements

Change-Id: I555936f97126ba59443e7f5ebb5286fc9489e3f9