py/obj: Add support for __int__ special method.

Based on the discussion, this special method is available unconditionally,
as converting to int is a common operation.

py/objint.c

@@ -62,8 +62,7 @@ STATIC mp_obj_t mp_obj_int_make_new(const mp_obj_type_t *type_in, size_t n_args,
                 return mp_obj_new_int_from_float(mp_obj_float_get(args[0]));
 #endif
             } else {
-                // try to convert to small int (eg from bool)
-                return MP_OBJ_NEW_SMALL_INT(mp_obj_get_int(args[0]));
+                return mp_unary_op(MP_UNARY_OP_INT, args[0]);
             }
 
         case 2: