Add Python3 Train unit tests

This is one goal of Train Cycle.
See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: Id5a150cbfa658383f6ae7958a06ebe3a4f9047e6

.zuul.yaml

@@ -3,8 +3,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
-      - openstack-python37-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - openstackclient-plugin-jobs

setup.cfg

@@ -17,6 +17,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py36,py27,pep8
+envlist = py36,py37,py27,pep8
 skipsdist = True
 
 [testenv]