Update the constraints url

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

Change-Id: I357b434a30128b23c8e80bfe4ec4fbd4655d36cc
diff --git a/tox.ini b/tox.ini
index bba37bb..95352a2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14,7 +14,7 @@
 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}/test-requirements.txt
 commands = stestr run --slowest {posargs}