py: Rename MP_OBJ_NOT_SUPPORTED to MP_OBJ_NULL.

See issue #608 for justification.

py/objset.c

@@ -475,7 +475,7 @@ STATIC mp_obj_t set_unary_op(int op, mp_obj_t self_in) {
     switch (op) {
         case MP_UNARY_OP_BOOL: return MP_BOOL(self->set.used != 0);
         case MP_UNARY_OP_LEN: return MP_OBJ_NEW_SMALL_INT((machine_int_t)self->set.used);
-        default: return MP_OBJ_NOT_SUPPORTED;
+        default: return MP_OBJ_NULL; // op not supported
     }
 }
 
@@ -514,7 +514,7 @@ STATIC mp_obj_t set_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
             return MP_BOOL(elem != NULL);
         }
         default:
-            return MP_OBJ_NOT_SUPPORTED;
+            return MP_OBJ_NULL; // op not supported
     }
 }