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: Iab77d920bc5c46a7d7d21c05def60ba278b56983
Story: #2005924
Task: #34243

.zuul.d/project.yaml

@@ -2,7 +2,7 @@
     templates:
       - openstack-pypy-jobs-nonvoting
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
     check:
       jobs:
         - build-wheel-mirror-centos-7:

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 = 3.1.0
 skipsdist = True
-envlist = validate,py36,py27,pep8,pip-install
+envlist = validate,py27,py37,pep8,pip-install
 ignore_basepython_conflict=true
 
 [testenv]