Update requirements URLs in tox config
Update requirements URLs in tox config

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so will switch to the correct
upper-constraints list automatically when the requirements repository branches.

Change-Id: I496dfbc709ecd864f619618252ef5dc992216ca7

openstack/python-neutronclient,openstack/python-neutronclient