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: I4379a60f3e6a511b714a4df1502d068bc1aaf6a3
Story: #2005924
Task: #34253

setup.cfg

@@ -15,6 +15,11 @@ classifier =
 	License :: OSI Approved :: Apache Software License
 	Operating System :: OS Independent
 	Programming Language :: Python
+	Programming Language :: Python :: 2
+	Programming Language :: Python :: 2.7
+	Programming Language :: Python :: 3
+	Programming Language :: Python :: 3.6
+	Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

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

zuul.d/layout.yaml

@@ -4,6 +4,5 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - tripleo-multinode-container-minimal