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: I3cb88c079088916164d82a0f5b253c22528c0543
Story: #2005924
Task: #34234

.zuul.yaml

@@ -5,9 +5,7 @@
       - lib-forward-testing-python3
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

setup.cfg

@@ -14,7 +14,8 @@ 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
     Topic :: Software Development :: Libraries
 
 [global]

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,docs,pep8
+envlist = py27,py37,docs,pep8
 
 [testenv:docs]
 basepython = python3