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

Depends-On: https://review.opendev.org/669542

Change-Id: I7add04a3f3d02c5e771088986920b6743234dfee
Story: #2005924
Task: #34235

.zuul.yaml

@@ -2,6 +2,5 @@
     templates:
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti

setup.cfg

@@ -10,7 +10,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Intended Audience :: Developers
     License :: OSI Approved :: Apache Software License
     Topic :: Software Development :: Libraries

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py27,pypy,pep8
+envlist = py27,py37,pypy,pep8
 minversion = 2.0
 skipsdist = True