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: I0362accbea950ce67401872716df393b416a72db
Story: #2005924
Task: #34199

.zuul.yaml

@@ -3,7 +3,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs-horizon
       - openstack-python-jobs-horizon
-      - openstack-python36-jobs-horizon
+      - openstack-python3-train-jobs-horizon
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -17,6 +17,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py27,pep8
+envlist = py27,py36,py37,pep8
 minversion = 2.0
 skipsdist = True