modbuiltins: Add NotImplemented builtin constant.

From https://docs.python.org/3/library/constants.html#NotImplemented :
"Special value which should be returned by the binary special methods
(e.g. __eq__(), __lt__(), __add__(), __rsub__(), etc.) to indicate
that the operation is not implemented with respect to the other type;
may be returned by the in-place binary special methods (e.g. __imul__(),
__iand__(), etc.) for the same purpose. Its truth value is true."

Some people however appear to abuse it to mean "no value" when None is
a legitimate value (don't do that).

py/modbuiltins.c

@@ -634,6 +634,9 @@ STATIC const mp_map_elem_t mp_module_builtins_globals_table[] = {
 
     // built-in objects
     { MP_OBJ_NEW_QSTR(MP_QSTR_Ellipsis), (mp_obj_t)&mp_const_ellipsis_obj },
+    #if MICROPY_PY_BUILTINS_NOTIMPLEMENTED
+    { MP_OBJ_NEW_QSTR(MP_QSTR_NotImplemented), MP_OBJ_SENTINEL },
+    #endif
 
     // built-in user functions
     { MP_OBJ_NEW_QSTR(MP_QSTR_abs), (mp_obj_t)&mp_builtin_abs_obj },