Update PEP 669 specification to return current callback object when r…

…egistering a new callback. (#2246)

pep-0669.rst

@@ -127,6 +127,8 @@ To register a callable for events call::
 
   sys.monitoring.register_callback(event, func)
 
+``register_callback`` returns the previously registered callback, or ``None``.
+
 Functions can be unregistered by calling
 ``sys.monitoring.register_callback(event, None)``.
 
@@ -192,7 +194,7 @@ Attributes of the ``sys.monitoring`` namespace
 * ``def set_events(event_set: Event)->None``
 * ``def get_local_events(code: CodeType)->Event``
 * ``def set_local_events(code: CodeType, event_set: Event)->None``
-* ``def register_callback(event: Event, func: Callable)->None``
+* ``def register_callback(event: Event, func: Callable)->Optional[Callable]``
 * ``def insert_marker(code: CodeType, offset: Event, marker_id=0: range(256))->None``
 * ``def remove_marker(code: CodeType, offset: Event)->None``