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: Iea75ba67e7f5c65c066b8aceb8509aec0b1f3442

tox.ini

@@ -8,7 +8,7 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 
@@ -73,7 +73,7 @@ exclude = .venv,.tox,dist,doc,*egg,releasenotes
 
 [testenv:releasenotes]
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html