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: I56899805c2492c7002e35c3fb8a078e68fb0ed8e
Story: #2005924
Task: #34247

.zuul.yaml

@@ -3,8 +3,8 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs-horizon
-      - openstack-python36-jobs-horizon
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs-horizon
+      - openstack-python3-train-jobs
     check:
       jobs:
         - horizon-openstack-tox-python3-django111

setup.cfg

@@ -9,6 +9,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Environment :: OpenStack
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py36,py27,pep8
+envlist = py27,py37,pep8
 
 [testenv]
 usedevelop = True