[goal] Migrate tox based testing to Focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Fixing pep8 job.

Stable jobs testing stable branch needs to keep running on their supported
distro version which is bionic from stein till ussuri.

Depends-On: https://review.opendev.org/#/c/738328/

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html

Change-Id: I6cc893ee3cb24afbdc73144213f9d059cec54df3
Story: #2007865
Task: #40186
2 files changed