python/cpython#8449 is a fix for a bug that doesn't appear in Python 3. There's no corresponding PR for master. But, Bedevere's backport check sees a PR for a maintenance branch, and insists that there should be a (GH-XXX) note in the title.

I didn't find guidance for this specific case in the devguide, but I think the correct thing to do is to merge without the (GH-XXX) note, ignoring the status check.