Update TOX_CONSTRAINTS_FILE for stable/xena

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

Change-Id: I26944288a56c226a3f217d5d2bae6d23181fdcac

tox.ini

@@ -6,7 +6,7 @@ ignore_basepython_conflict = true
 
 [testenv]
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
   -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
@@ -25,7 +25,7 @@ commands = python setup.py test --coverage --coverage-package-name=oslo_versione
 [testenv:docs]
 allowlist_externals = rm
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -fr doc/build