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: I29a89ee1abbbfaa7d9593923c17f8999b9323d25
Story: #2005924
Task: #34215

.zuul.yaml

@@ -1,7 +1,7 @@
 - project:
     templates:
       - openstack-specs-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
     check:
       jobs:
         - openstack-tox-py27

setup.cfg

@@ -10,6 +10,11 @@ classifier =
     Intended Audience :: Developers
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
+    Programming Language :: Python
+    Programming Language :: Python :: 2
+    Programming Language :: Python :: 2.7
+    Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.7
 
 [build_sphinx]
 all_files = 1

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = docs,py27
+envlist = docs,py27,py37
 skipsdist = True
 
 [testenv]