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: I764b9765484e1d8217d56796d984f910aa5f9c5a
Story: #2005924
Task: #34249

.zuul.yaml

@@ -39,9 +39,7 @@
       - openstack-lower-constraints-jobs
       - openstack-pypy-jobs-nonvoting
       - 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

@@ -17,7 +17,6 @@ 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
 

tox.ini

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