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: I41f1b540e3e54ddc434b4e0913da45afea0e2b63
Story: #2005924
Task: #34225

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 = py36,py27,pep8,docs
+envlist = py27,py37,pep8,docs
 skipsdist = True
 
 [testenv]

zuul.d/project.yaml

@@ -4,8 +4,7 @@
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
       - check-requirements
-      - openstack-python36-jobs-neutron
-      - openstack-python37-jobs-neutron
+      - openstack-python3-train-jobs-neutron
       - openstack-lower-constraints-jobs-neutron
     check:
       jobs: