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: I8b8ab63cbfe7dced5e1d79d768ee2b8f11a464bb
Story: #2005924
Task: #34223

.zuul.yaml

@@ -4,4 +4,4 @@
       - openstack-cover-jobs-horizon
       - openstack-lower-constraints-jobs
       - openstack-python-jobs-horizon
-      - openstack-python36-jobs-horizon
+      - openstack-python3-train-jobs-horizon

setup.cfg

@@ -16,6 +16,7 @@ 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 = py27,pep8,py36
+envlist = py27,py37,pep8
 minversion = 2.6
 skipsdist = True