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: I29cc4548f827d0ce5428731a31f8da5d52aae559
Story: #2005924
Task: #34232

.zuul.yaml

@@ -46,5 +46,4 @@
       - build-openstack-docs-pti
       - check-requirements
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs

setup.cfg

@@ -16,8 +16,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.4
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [build_sphinx]
 source-dir = doc/source

tox.ini

@@ -1,6 +1,6 @@
 [tox]
-minversion = 1.6
-envlist = py35,py27,pep8,docs
+minversion = 3.1.1
+envlist = py27,py37,pep8,docs
 
 [testenv]
 deps = -r{toxinidir}/test-requirements.txt