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/#/c/598678/4
Change-Id: I5ff38cd614f448ede127961a9f8d133a638d7e89
Story: #2002586
Task: #24759

.zuul.yaml

@@ -2,6 +2,7 @@
     templates:
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
     check:
       jobs:
         - refstack-client-devstack-tempestconf: