[core-workflow] "backport needed" status

If there is objection I will happily revert it, but I needed to mark an issue on bugs.python.org as still needing backport, since I'd merged the fix and the PR is now closed but I have no time currently to learn about how to do backports(*). So I added a 'backport needed' status to the tracker. I there is agreement that this is a good idea and should remain, we can maybe get rid of it later after backports are automated. But I have a feeling we'll want it long term, given the new workflow and the fact that not all backports can be done automatically. --David (*) Or, to be more accurate, I'm waiting until you all figure out how to automate it.