issue2898 smells like an implementation detail to me. Also, sys.getsizeof() isn't necessarily authoritative here, since the sys module is full of all sorts of implementation details.

I'm not saying __sizeof__ shouldn't be made an official part of the language, but we should be careful before we effectively etch something in stone for all Python implementations.  It may be worth getting feedback from python-dev, particularly from the major Python implementers.