Update TOX_CONSTRAINTS_FILE for stable/yoga

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/yoga branch, tests will
continue to use the upper-constraints list on master.

Change-Id: Ie2d096d324ffceb9f63cc0e727516c35602b6698

tox.ini

@@ -6,7 +6,7 @@ ignore_basepython_conflict = true
 [testenv]
 basepython = python3
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands =
@@ -26,7 +26,7 @@ commands = {posargs}
 
 [testenv:docs]
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
   -r{toxinidir}/doc/requirements.txt
 allowlist_externals = rm
 commands =