Message4630

In some places PySequence (and IIRC some other places) assume __len__ 
returns the real length of a sequence. That's not always the case for 
derived objects. This is bound to break something at some point

Most places (outside of PySequence) use the real size when they should 
(e.g. PyList/Tuple/String call size/length on themselves instead of 
__len__ when needed) but the base PySequence doesn't have that luxury. 
It should have a size() method it can rely on