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

Note that python35-charm-jobs is retained since this charm is supported
on Xenial.

Change-Id: I10bcd0c4847e2f0828c8df6d23d148d2b55c74f7
Story: #2005924
Task: #34228

.zuul.yaml

@@ -1,3 +1,4 @@
 - project:
     templates:
       - python35-charm-jobs
+      - openstack-python3-train-jobs