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: Ic60993c2315e8b10da8abb1fb02e33151a1e2b98

.zuul.yaml

@@ -2,6 +2,6 @@
     templates:
       - check-requirements
       - openstack-python-jobs-ceilometer
-      - openstack-python36-jobs-ceilometer
+      - openstack-python3-train-jobs-ceilometer
       - periodic-stable-jobs-ceilometer
       - openstack-lower-constraints-jobs-ceilometer

setup.cfg

@@ -15,7 +15,7 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [entry_points]
 ceilometer.compute.virt =

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.1.1
 skipsdist = True
-envlist = py36,py27,pep8
+envlist = py27,py37,pep8
 # Automatic envs (pyXX) will use the python version appropriate to that
 # env and ignore basepython inherited from [testenv]. That's what we
 # want, and we don't need to be warned about it.