PL/Python result set slicing broken in Python 3

This doesn't work anymore with Python 3:

rv = plpy.execute(...)
do_something(rv[0:1])

Apparently, they changed the C API for doing slicing, or rather made one
of the two APIs for it silently do nothing.  Details are difficult to
find, but this email message seems to contain something:
<http://mail.python.org/pipermail/python-3000/2007-August/009851.html>.

I'll try to sort this out sometime, but if someone wants to take a shot
at it, go ahead.