Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: I63ac91eecc784a935a6be27ab95a2e34e36f7c56
Story: #2005924
Task: #34245

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - openstack-specs-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - check-requirements