str subclasses may have non-empty __slots__, bytes subclasses can't.

Doc/reference/datamodel.rst

@@ -1571,7 +1571,7 @@ Notes on using *__slots__*
   program undefined.  In the future, a check may be added to prevent this.
 
 * Nonempty *__slots__* does not work for classes derived from "variable-length"
-  built-in types such as :class:`int`, :class:`str` and :class:`tuple`.
+  built-in types such as :class:`int`, :class:`bytes` and :class:`tuple`.
 
 * Any non-string iterable may be assigned to *__slots__*. Mappings may also be
   used; however, in the future, special meaning may be assigned to the values