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: I968b523f20edb5e9900c99bd919a9a5ede324eac
Story: #2005924
Task: #34221

.zuul.yaml

@@ -3,7 +3,6 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

setup.cfg

@@ -16,8 +16,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = pep8,py27,py35,py36
+envlist = pep8,py27,py36,py37
 skipsdist = True
 
 [testenv]
@@ -26,14 +26,14 @@ commands =
     {[testenv]commands}
     stestr run {posargs}
 
-[testenv:py35]
-basepython = python3
+[testenv:py36]
+basepython = python3.6
 commands =
     {[testenv]commands}
     stestr run {posargs}
 
-[testenv:py36]
-basepython = python3.6
+[testenv:py37]
+basepython = python3.7
 commands =
     {[testenv]commands}
     stestr run {posargs}