Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Note: This diverges from the Train goal as it does not drop the
openstack-python35-jobs. As tempest is branchless it needs to continue
supporting Python 3.5.

Change-Id: Ie702ca637afb8229b6e605abcec25550bc9a0673
Story: #2005924
Task: #34240
2 files changed