* in/out make sense from a C perspective but when you get to the
   python layers it makes more sense to label them as to_py and
   from_py to denote which way we are marshalling
 * this helps clear up the difference between callbacks which
   call into python and invoked functions which call into C
 * in the callback case we marshal in values to Python objects
   and out values to C types but in the invoke case we do the
   reverse.  Dealing with to_py/from_py makes the code much more
   resuable and consistant