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: Iac73937b60a6f8e7520123fbf59e748d296d4c7d

switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder