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: I439f39b525158d8a6936bb0498c8d818bc1ef36d
Story: #2002586
Task: #24286

.zuul.yaml

@@ -3,6 +3,7 @@
       - check-requirements
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
     check:
       jobs:
         - openstack-tox-lower-constraints:
@@ -17,6 +18,9 @@
         - openstack-tox-py35:
             required-projects:
               - openstack/nova
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/nova
     gate:
       jobs:
         - openstack-tox-lower-constraints:
@@ -31,3 +35,6 @@
         - openstack-tox-py35:
             required-projects:
               - openstack/nova
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/nova