[3.12] Clarify state of CancelledError in doc (GH-106453) (#106454)

Clarify state of CancelledError in doc (GH-106453)

This change makes it explicit that asyncio.CancelledError is not a subclass of Exception.
(cherry picked from commit 12a9813)

Co-authored-by: Kristján Valur Jónsson <sweskman@gmail.com>

Doc/library/asyncio-exceptions.rst

@@ -31,7 +31,7 @@ Exceptions
 
    .. versionchanged:: 3.8
 
-      :exc:`CancelledError` is now a subclass of :class:`BaseException`.
+      :exc:`CancelledError` is now a subclass of :class:`BaseException` rather than :class:`Exception`.
 
 
 .. exception:: InvalidStateError