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: Ie2119dec469dba059fa3ce8729f360590d45791c
Story: #2005924
Task: #34204

.zuul.yaml

@@ -5,8 +5,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti
     gate:

setup.cfg

@@ -18,6 +18,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Topic :: Internet :: Name Service (DNS)
 
 [global]

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py27,flake8
+envlist = py27,py37,flake8
 minversion = 2.0
 skipsdist = True