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: I1f280a05e3d329abf07c9d50ae28aee602691369

.zuul.yaml

@@ -3,5 +3,5 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs-neutron
-      - openstack-python36-jobs-neutron
+      - openstack-python3-train-jobs-neutron
       - periodic-stable-jobs-neutron

lower-constraints.txt

@@ -39,6 +39,7 @@ keystonemiddleware==4.17.0
 kombu==4.0.0
 linecache2==1.0.0
 logutils==0.3.5
+libxml2-python3==2.9.5
 lxml==3.4.1
 Mako==0.4.0
 MarkupSafe==1.0

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]
 console_scripts =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py27,pep8
+envlist = py27,py37,pep8
 minversion = 3.1.1
 skipsdist = True
 # Automatic envs (pyXX) will use the python version appropriate to that