Update the constraints url

For more detail, see http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html

Change-Id: Ib56288ab747cd4f6999681bf978934a21fdf012a
diff --git a/tox.ini b/tox.ini
index f32dd68..00b23d0 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,7 +5,7 @@
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning