Add constraints to stable/newton

cmd2 has dropped py2 support but the upper-constraints.txt will prevent
the inclusion of the new version. We should use the stable constraints
for testing.

Change-Id: If4284277f3e634b9441205a07edd359223768ff2
Related-Bug: #1773936

tox.ini

@@ -6,8 +6,11 @@ envlist = py34,py27,pep8
 [testenv]
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
-       -r{toxinidir}/requirements.txt
+install_command = pip install {opts} {packages}
+deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/newton}
+    -r{toxinidir}/test-requirements.txt
+    -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]