Use Py_RETURN_NONE where suitable

This is more idiomatic style and available as of Python 2.4, which is
our minimum.

src/pl/plpython/plpy_cursorobject.c

@@ -509,6 +509,5 @@ PLy_cursor_close(PyObject *self, PyObject *unused)
 		cursor->closed = true;
 	}
 
-	Py_INCREF(Py_None);
-	return Py_None;
+	Py_RETURN_NONE;
 }