PL/Python: Fix slicing support for result objects for Python 3

The old way of implementing slicing support by implementing
PySequenceMethods.sq_slice no longer works in Python 3.  You now have
to implement PyMappingMethods.mp_subscript.  Do this by simply
proxying the call to the wrapped list of result dictionaries.
Consolidate some of the subscripting regression tests.

Jan Urbański