Update the constraints url

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

Change-Id: I2bf2fcc1ddb5fbda4486da439773e4c275376a27
diff --git a/tox.ini b/tox.ini
index f1581bc..88bb805 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,7 +9,7 @@
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 setenv =