More on SF bug [#460020] bug or feature: unicode() and subclasses.

tuple(i) repaired to return a true tuple when i is an instance of a
tuple subclass.
Added PyTuple_CheckExact macro.
PySequence_Tuple():  if a tuple-like object isn't exactly a tuple, it's
not safe to return the object as-is -- make a new tuple of it instead.

Include/tupleobject.h

@@ -27,6 +27,7 @@ typedef struct {
 extern DL_IMPORT(PyTypeObject) PyTuple_Type;
 
 #define PyTuple_Check(op) PyObject_TypeCheck(op, &PyTuple_Type)
+#define PyTuple_CheckExact(op) ((op)->ob_type == &PyTuple_Type)
 
 extern DL_IMPORT(PyObject *) PyTuple_New(int size);
 extern DL_IMPORT(int) PyTuple_Size(PyObject *);