Replace deprecated UPPER_CONSTRAINTS_FILE variable

UPPER_CONSTRAINTS_FILE is deprecated and TOX_CONSTRAINTS_FILE is
the new environment variable name that replaces it [1].

This allows to use upper-constraints file as more
readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

[1] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file
[2] https://review.opendev.org/#/c/722814/

Change-Id: I362ea7f5a3064892503b6e8e4d663f72ac23bea3

tox.ini

@@ -9,7 +9,7 @@ basepython = python3
 setenv =
    VIRTUAL_ENV={envdir}
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE: https://releases.openstack.org/constraints/upper/master}
+       -c{env:TOX_CONSTRAINTS_FILE: https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}