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

Co-Authored-By: gujin <gu.jin@99cloud.net>
Change-Id: I4585d250117f19cca577b2e3aaa74aba7ec73e9b
Story: #2005924
Task: #34221

.zuul.yaml

@@ -3,6 +3,6 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

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 = pep8,py27,py36
+envlist = pep8,py27,py36,py37
 skipsdist = True
 
 [testenv]
@@ -34,6 +34,12 @@ commands =
     {[testenv]commands}
     stestr run {posargs}
 
+[testenv:py37]
+basepython = python3.7
+commands =
+    {[testenv]commands}
+    stestr run {posargs}
+
 [testenv:genconfig]
 basepython = python3
 commands = oslo-config-generator --config-file=etc/masakarimonitors/masakarimonitors-config-generator.conf