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: I1681490fcaa52593abcbf8f935cbe1df7769f2eb
Story: #2002586
Task: #24320

Author: QingsZhao

.zuul.yaml

@@ -194,6 +194,7 @@
       - osc-tox-unit-tips
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3