The 2.6 and 3.0 documentation has already been updated appropriately.
The forward compatible way to handle this is to inherit from object and
override the special methods explicitly in the 2.x version (Yes this can
make writing proxy objects more tedious, but from our experience with
the tempfile module, I would say that the bulk of proxy objects that
aren't overriding special methods on a case-by-case basis are probably
broken anyway).

It may be appropriate to add a -3 warning that is triggered whenever a
special method is retrieved via __getattr__ on a classic class.