Update TOX_CONSTRAINTS_FILE for stable/zed

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

Until the requirements repository has as stable/zed branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I8e54b33e10f2ec2dbcc02fcc4456c0c988a929df

tox.ini

@@ -10,7 +10,7 @@ setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_NOCAPTURE=False
          OS_STDERR_NOCAPTURE=False
 deps =
-    -c{env:TOX_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
     -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}
@@ -70,7 +70,7 @@ setenv =
     {[testenv]setenv}
     SPHINX_APIDOC_OPTIONS=noindex,members,undoc-members,show-inheritance
 deps =
-    -c{env:TOX_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
     -r{toxinidir}/doc/requirements.txt
 commands =
     bash -c "rm -rf doc/build"
@@ -93,7 +93,7 @@ commands =
 
 [testenv:releasenotes]
 deps =
-    -c{env:TOX_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
     -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html