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: Icce6506a61590240e8a320146ef2648837173ca1
Story: #2005924
Task: #34214

.zuul.yaml

@@ -1,6 +1,5 @@
 - project:
     templates:
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - check-requirements

setup.cfg

@@ -16,7 +16,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py27,py27dj18,pep8
+envlist = py27,py37,py27dj18,pep8
 minversion = 1.6
 skipsdist = True