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: I0579d9293f8969ecf2f8b813b8419d1e31e34bee
Story: #2002586
Task: #24291

.zuul.yaml

@@ -4,5 +4,6 @@
       - publish-openstack-docs-pti
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
       - lib-forward-testing
       - release-notes-jobs-python3