Switch to TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is old name and deprecated
-https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file
This allows to use lower-constraints file as more
readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

Also stable ussuri branch is also switching to new var so
it make sense to may the master also sync with that.
- https://review.opendev.org/#/c/722520/

Change-Id: I3ce0bdc2b928ec63161cf0fa4757c43b83f2af09

tox.ini

@@ -26,7 +26,7 @@ setenv =
 # TODO(stephenfin): Remove psycopg2 when minimum constraints is bumped to 2.8
   PYTHONWARNINGS = ignore::UserWarning:psycopg2
 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
 passenv =
@@ -157,7 +157,7 @@ description =
 # Note that we don't use {[testenv]deps} for deps here because we don't want
 # to install (test-)requirements.txt for docs.
 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}/doc/requirements.txt
 commands =
   rm -rf doc/build/html doc/build/doctrees