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

Depends-On: https://review.openstack.org/596861
Change-Id: I5a361dc3b651399042174f183c5c2f46bc26aea1
Story: #2002586
Task: #24308

.zuul.yaml

@@ -267,6 +267,7 @@
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
       - check-requirements
       - publish-openstack-docs-pti
       - release-notes-jobs-python3