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: Id4099c1e2384fd431a0b3f3d831c2437d7761e1e
Story: #2005924
Task: #34223

.zuul.yaml

@@ -4,7 +4,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
     check:
       jobs:
         - monasca-tempest-python2-influxdb

setup.cfg

@@ -13,7 +13,9 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 2.7
+    Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [global]
 setup-hooks =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,36},pep8,cover
+envlist = py27,py37,pep8,cover
 minversion = 2.5
 skipsdist = True