Make StopIteration a sink state. This is done by clearing out the

Make StopIteration a sink state.  This is done by clearing out the
object references (it_seq for seqiterobject, it_callable and
it_sentinel for calliterobject) when the end of the list is reached.

Also remove the next() methods -- one is supplied automatically by
PyType_Ready() because the tp_iternext slot is set.  That's a good
thing, because the implementation given here was buggy (it never
raised StopIteration).