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: I7234a0d87b66095a2acabbb0f17cbc0438de0ee0
Story: #2005924
Task: #34214

.zuul.yaml

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

setup.cfg

@@ -21,6 +21,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

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