The new stable upper-constraints file is only available
after the openstack/requirements repository is branched.
This will happen around the RC1 timeframe.
Recheck and merge this change once the requirements
repository has been branched.
The CI system will work with this patch before the requirements
repository is branched because zuul configues the job to run
with a local copy of the file and defaults to the master branch.
However, accepting the patch will break the test configuration
on developers' local systems, so please wait until after the
requirements repository is branched to merge the patch.