Add Python 3 Train unit tests

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

Change-Id: Ic509bdfc23d95d709db8b534827ab141e3d2a0b0

.zuul.yaml

@@ -25,8 +25,7 @@
     templates:
       - horizon-nodejs10-jobs-nonvoting
       - openstack-python-jobs-horizon
-      - openstack-python36-jobs-horizon
-      - openstack-python37-jobs-horizon
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3