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: I5c93f7e942b1b73023e4e26cdf4a3e5ea957c925changes/83/761383/2