Use generic attribute management in PL/Python

Switch the implementation of the plan and result types to generic attribute
management, as described at <http://docs.python.org/extending/newtypes.html>.
This modernizes and simplifies the code a bit and prepares for Python 3.1,
where the old way doesn't work anymore.

src/pl/plpython/expected/plpython_spi.out

@@ -111,3 +111,24 @@ SELECT join_sequences(sequences) FROM sequences
 ----------------
 (0 rows)
 
+--
+-- plan and result objects
+--
+CREATE FUNCTION result_nrows_test() RETURNS int
+AS $$
+plan = plpy.prepare("SELECT 1 UNION SELECT 2")
+plpy.info(plan.status()) # not really documented or useful
+result = plpy.execute(plan)
+if result.status() > 0:
+   return result.nrows()
+else:
+   return None
+$$ LANGUAGE plpythonu;
+SELECT result_nrows_test();
+INFO:  (True,)
+CONTEXT:  PL/Python function "result_nrows_test"
+ result_nrows_test 
+-------------------
+                 2
+(1 row)
+