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

Note that python35-charm-jobs is retained since this charm is supported
on Xenial.

Change-Id: I05e9172cc4c84f76e110b467ff311bed71aa6aab
Story: #2005924
Task: #34228

.zuul.yaml

@@ -1,4 +1,5 @@
 - project:
     templates:
       - python35-charm-jobs
+      - openstack-python3-train-jobs
       - openstack-cover-jobs

setup.cfg

@@ -11,6 +11,11 @@ classifier =
     Intended Audience :: System Administrators
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
+    Programming Language :: Python
+    Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [nosetests]
 verbosity=1

tox.ini

@@ -2,7 +2,7 @@
 # This file is managed centrally by release-tools and should not be modified
 # within individual charm repos.
 [tox]
-envlist = pep8,py27,py35
+envlist = pep8,py27,py37
 skipsdist = True
 
 [testenv]