Use TOX_CONSTRAINTS_FILE

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

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

Change-Id: I9a48a44a1cc32c3ab9393b97ea9014dc95cfd4de

tox.ini

@@ -14,7 +14,7 @@ install_command = pip install {opts} {packages}
 # Install also sitemap scraping tool, not installed by default
 # therefore not in requirements file
 deps = scrapy>=1.0.0
-       -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}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}