add python 3.6 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.6 as part of the python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: I300eb6cdd44d5486e5eb4cc4bf9b05aa3678251f
Story: #2002586
Task: #24763
1 file changed