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: Ib55d9c890de62bdfe269390c5533144e46e92703
Story: #2005924
Task: #34233

.zuul.yaml

@@ -396,8 +396,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - openstacksdk-functional-tips
       - openstacksdk-tox-tips
       - os-client-config-tox-tips

setup.cfg

@@ -16,8 +16,8 @@ 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
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1
-envlist = pep8,py37,py36,py35,py27
+envlist = pep8,py27,py37
 skipsdist = True
 ignore_basepython_conflict = True