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: I16e5d68aa3a503fcf3ccad19572f3a451e032601
Story: #2002586
Task: #24342

.zuul.yaml

@@ -8,9 +8,23 @@
         - openstack-tox-py35:
             required-projects:
               - openstack/horizon
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/horizon
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/horizon
     gate:
       jobs:
         - openstack-tox-py35:
             required-projects:
               - openstack/horizon
 
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/horizon
+
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/horizon
+