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: I35a537d0680c54fa995919f745cf4fe8b04d2543
Story: #2002586
Task: #24322

.zuul.yaml

@@ -13,3 +13,4 @@
       - lib-forward-testing
       - release-notes-jobs-python3
       - periodic-stable-jobs
+      - openstack-python36-jobs