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

Note that since tempest is branchless that we retain py35 support.

Change-Id: I622ef0306ac48509c46e243c83f458e57e496258
Story: #2005924
Task: #34257

.zuul.yaml

@@ -2,7 +2,7 @@
     templates:
       - openstack-python-jobs
       - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - check-requirements
     gate:
       queue: os-win

setup.cfg

@@ -19,6 +19,7 @@ classifier =
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,py36,py27,pypy,pep8
+envlist = py27,py37,pypy,pep8
 skipsdist = True
 
 [testenv]