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: Ib7b08bdd5ab6387157ab0f09acc516ffb7ea0dcb

.zuul.yaml

@@ -1,7 +1,7 @@
 - project:
     templates:
       - build-openstack-docs-pti
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
     check:
       jobs:
         - openstack-tox-linters: