As a user of Python 3, I would like echo Victor's comment about fixing the API right now as opposed to having to deal with it later.  I can only speak for myself, but I would guess that anyone using Python 3 already understands that it's bleeding edge and that the bytes/strings distinction is really important.  If fixing this breaks some third party libraries, I say good--they shouldn't have been blindly passing Unicode into struct in the first place.  Better to deal with it now when the number of users is relatively small.