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: Iede631bcdd1dac0789775b6ffe3ae52834339bd5
Story: #2005924
Task: #34240

.zuul.yaml

@@ -4,7 +4,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - openstack-pypy-jobs-nonvoting
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -19,6 +19,7 @@ classifier =
     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 = py{36,27},pep8,pypy
+envlist = py27,py36,py37,pep8,pypy
 
 [testenv]
 usedevelop = True