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: Ieb9ecac8017b2a6a4b5d20ca9beb36160eb6e602

tox.ini

@@ -8,7 +8,7 @@ basepython = python3
 usedevelop = True
 allowlist_externals = rm
 deps =
-       -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
+       -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 setenv = VIRTUAL_ENV={envdir}
@@ -39,7 +39,7 @@ commands = flake8 {posargs}
 commands = {posargs}
 
 [testenv:docs]
-deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
 commands =