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: Ida4cffc0ed45bc3a4bb11874b48063218c287e8a
Story: #2005924
Task: #34246

.zuul.yaml

@@ -4,7 +4,7 @@
       - horizon-nodejs10-jobs
       - 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

@@ -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
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27-local,py36-local,py3-dj111-local,eslint,karma-local,docs-local,releasenotes,lower-constraints-local
+envlist = py37,pep8,py27-local,py36-local,py3-dj111-local,eslint,karma-local,docs-local,releasenotes,lower-constraints-local
 minversion = 2.3.2
 skipsdist = True