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: I75230c0244dc9f32eacd388c4bcc4d7b27e53f74
Story: #2005924
Task: #34199

.zuul.yaml

@@ -47,9 +47,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -8,7 +8,8 @@ classifiers =
     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
     Environment :: OpenStack
     Development Status :: 3 - Alpha
     Framework :: Setuptools Plugin

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py27,pep8
+envlist = py27,py36,py37,pep8
 minversion = 2.0
 skipsdist = True