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: Ie76efa2dad81f1147250c4f6e56d6bd3c1db0869
Story: #2005924
Task: #34239

.zuul.yaml

@@ -2,7 +2,7 @@
     templates:
       - check-requirements
       - openstack-python-jobs-horizon
-      - openstack-python35-jobs-horizon
+      - openstack-python3-train-jobs-horizon
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -16,9 +16,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.3
-    Programming Language :: Python :: 3.4
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py3-dj111,pep8,docs
+envlist = py27,py37,py3-dj111,pep8,docs
 minversion = 2.3.2
 skipsdist = True