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: I33a6c580372de6a40f8d01fa4117415528bd5782
Story: #2002586
Task: #24341

zuul.d/layout.yaml

@@ -5,6 +5,7 @@
       - check-requirements
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
     check:
       jobs:
         - openstack-tox-cover: