pitrou: You're missing a few steps.  If you are maintaining project X,
which depends on library Y, that adds a showwarning hook for some
reason, you need to:

 * check out a development version of library Y, which you do not
normally maintain
 * change the hook's signature to the new one
 * submit a patch to library Y
 * make a local fork of library Y so that you can verify the rest of
your code under 2.6
 * send out an email to your mailing list explaining that they cannot
run with python 2.6 without this patch (linking to the patch in library Y)
 * write a FAQ entry because 200 people show up asking why it does not
work on python 2.6
 * if library Y's development is slow, do a new release of project X
which includes monkeypatching to allow running project X on 2.6

These steps may be unavoidable if library Y has been unmaintained for a
long enough time, but it would be nice to be able to avoid this with a
single new Python release.