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: I3812281ad016d0d650fad44e41ea4c83df11fb89
Story: #2005924
Task: #34253

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = pep8,py37,py36,py35,py27
+envlist = pep8,py27,py37
 
 [testenv]
 usedevelop = True

zuul.d/layout.yaml

@@ -2,7 +2,5 @@
     templates:
       - tripleo-buildimage-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - release-notes-jobs-python3