- A new type object, 'string', is added. This is a common base type

for 'str' and 'unicode', and can be used instead of
  types.StringTypes, e.g. to test whether something is "a string":
  isinstance(x, string) is True for Unicode and 8-bit strings.  This
  is an abstract base class and cannot be instantiated directly.

Python/bltinmodule.c

@@ -1905,6 +1905,7 @@ _PyBuiltin_Init(void)
 	SETBUILTIN("object",		&PyBaseObject_Type);
 	SETBUILTIN("staticmethod",	&PyStaticMethod_Type);
 	SETBUILTIN("str",		&PyString_Type);
+	SETBUILTIN("string",		&PyBaseString_Type);
 	SETBUILTIN("super",		&PySuper_Type);
 	SETBUILTIN("tuple",		&PyTuple_Type);
 	SETBUILTIN("type",		&PyType_Type);