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: Icb362afded64c43ef51fdedf2fd2b054ef5e6218
Story: #2005924
Task: #34240

.zuul.yaml

@@ -11,8 +11,7 @@
       - nodejs8-jobs
       - openstack-python-jobs
       - publish-openstack-docs-pti
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
-      - openstack-python37-jobs
     periodic:
       jobs:
         - periodic-package-stackviz-element