In python/cpython#3548, the pr backports cleanly without conflict, but the test suite failed.

Some ideas of what @miss-islington can do:

• notify the original PR author, or the core dev merging the PR, that the backport failed (leave a comment)
• automatically close the backport PR? because it now requires manual backport. A core dev can close it too.
• re-add the needs backport to [X.Y] label to the original PR