[core-workflow] "backport needed" status

[R. David Murray]

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.