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: I41cbb002ac07dde1380c1edc6b05495aac67f04a
Story: #2005924
Task: #34240

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - openstack-cover-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - release-notes-jobs-python3
       - publish-openstack-docs-pti

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py{37,36,27},pep8,pip-check-reqs
+envlist = py37,pep8,pip-check-reqs
 skipsdist = True
 
 [testenv]