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: Ibc4be4c59c2dca995f1999879ea2a9e70b129f7f
Story: #2005924
Task: #34199

.zuul.yaml

@@ -3,6 +3,5 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - release-notes-jobs-python3

setup.cfg

@@ -9,6 +9,7 @@ classifiers =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Environment :: OpenStack
     Development Status :: 3 - Alpha
     Framework :: Setuptools Plugin

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py36,py34,py27,pep8
+envlist = py27,py36,py37,pep8
 
 [testenv]
 install_command = pip install {opts} {packages}