Backport fails with no explaination, but work if retried

[vstinner]

On my PR,https://github.com/python/cpython/pull/22281, I requested 3.8 and 3.9 backports.

At the first attempt, the bot succeeded to backport to 3.8, but failed to backport to 3.9.

At the second attempt, without touching the code or anything, the backport to 3.9 worked.

Who can investigate why the first attempt to backport to 3.9 failed? Can it be caused by the rate limit issue #153?

Would it be possible to provide an error message, traceback or anything, to help to debug such issue?

If the bot could say "please retry later" if the bot considers that it should work "later", it would be helpful to say it. But I don't know if it's possible to distinguish the different causes of failure.