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: I01dc2774b950fe4da9d715b1df376c2e38f45687
Story: #2005924
Task: #34233

.zuul.yaml

@@ -1,8 +1,7 @@
 - project:
     templates:
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py36,py37,py27,pep8
+envlist = py27,py37,pep8
 skipsdist = True
 
 [testenv]