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: I1ba956644bdf7c10bd2b6780e556df4c70bceeab
Story: #2005924
Task: #34209

.zuul.yaml

@@ -3,8 +3,6 @@
       - 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
       - publish-to-pypi

setup.cfg

@@ -15,6 +15,9 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 2
     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 = py35,py27,pep8
+envlist = py27,py37,pep8
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}