Just for the record, David's comment at msg158136 is apposite:

> We've had trouble in the past with a conversion to new style class
> breaking people's code.  People are less likely to be subclassing
> ZipFile, though, so it is probably OK.

We just spent some time this morning fixing a bug in some internal software; the cause was precisely this unexpected change from old-style class to new-style class in a bugfix release. And indeed, we were subclassing ZipFile. :-(