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: I749a23bbed7074116fba2b3c06e617601e6d4620
Story: #2005924
Task: #34225

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = docs,py36,py37,py27,pep8
+envlist = docs,py27,py37,pep8
 minversion = 2.3.2
 skipsdist = True