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: Ife7bbcb1f5d4f766ebf9470e252a8c0cf520ee16
Story: #2005924
Task: #34251

.zuul.yaml

@@ -1,8 +1,7 @@
 - project:
     templates:
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -18,6 +18,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Topic :: System :: Monitoring
 
 [global]

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py{27}{,-mysql,-postgresql},functional,pep8
+envlist = py{27,37}{,-mysql,-postgresql},functional,pep8
 
 [testenv]
 # NOTE(tonyb): This project has chosen to *NOT* consume upper-constraints.txt