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: Ifb5d57e078d16bd1b15418f38750fc0ef2b571f7

.zuul.yaml

@@ -28,6 +28,7 @@
       - openstack-lower-constraints-jobs
       - openstack-python-jobs-horizon
       - openstack-python35-jobs-horizon
+      - openstack-python36-jobs-horizon
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check: