Updated openstack/openstack
Project: openstack/oslo.serialization  437eaf8db510a3263ce230ce0ae910388d480858

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: Ia182d834cf3156f16f851ee7524e1713dd47425d