Since I8f5338b7eee31f1b8bc3e783d10666e3092a6540 we have not been running
pep8 jobs in CI. This change fixes the issues introduced since then, and
adds the job back.
Unlike the original change, we are not using the openstack-python3-jobs
template since it would use py38 on this branch.
Change-Id: Ie15019f91b9e7b7f8cf97b628886f6b0beeb24fa
(cherry picked from commit 610433b11ba3819869a892489546d9f8b2b743c2)