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: I3e06562e38f9ee165024e09b95408d5593c0cdab
Story: #2005924
Task: #34220

.zuul.yaml

@@ -4,8 +4,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - check-requirements
       - 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
 
 
 [global]

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 distribute = False
-envlist = py27,pep8
+envlist = py27,py37,pep8
 minversion = 2.0
 skipsdist = True