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: I6c176d151189b2a27b8ad72c65038bf0676f787c
Story: #2002586
Task: #24330

.zuul.yaml

@@ -10,6 +10,7 @@
 - project:
     templates:
       - openstack-python35-jobs
+      - openstack-python36-jobs
     check:
       jobs:
         - openstack-tox-validate:
@@ -24,6 +25,10 @@
             irrelevant-files:
               - ^deliverables/.*$
               - ^doc/.*$
+        - openstack-tox-py36:
+            irrelevant-files:
+              - ^deliverables/.*$
+              - ^doc/.*$
         - openstack-tox-pep8:
             irrelevant-files:
               - ^deliverables/.*$
@@ -41,6 +46,10 @@
             irrelevant-files:
               - ^deliverables/.*$
               - ^doc/.*$
+        - openstack-tox-py36:
+            irrelevant-files:
+              - ^deliverables/.*$
+              - ^doc/.*$
     release-post:
       jobs:
         - tag-releases