SF bug [#468061] __str__ ignored in str subclass.

object.c, PyObject_Str:  Don't try to optimize anything except exact
string objects here; in particular, let str subclasses go thru tp_str,
same as non-str objects.  This allows overrides of tp_str to take
effect.

stringobject.c:
+ string_print (str's tp_print):  If the argument isn't an exact string
  object, get one from PyObject_Str.

+ string_str (str's tp_str):  Make a genuine-string copy of the object if
  it's of a proper str subclass type.  str() applied to a str subclass
  that doesn't override __str__ ends up here.

test_descr.py:  New str_of_str_subclass() test.

Objects/object.c

@@ -261,12 +261,6 @@ PyObject_Str(PyObject *v)
 		Py_INCREF(v);
 		return v;
 	}
-	if (PyString_Check(v)) {
-		/* For a string subtype that's not a string, return a true
-		   string with the same string data. */
-		PyStringObject *s = (PyStringObject *)v;
-		return PyString_FromStringAndSize(s->ob_sval, s->ob_size);
-	}
 	if (v->ob_type->tp_str == NULL)
 		return PyObject_Repr(v);