bpo-46829: Deprecate passing a message into Future.cancel() and Task.cancel() (GH-31840)

After a long deliberation we ended up feeling that the message argument for Future.cancel(), added in 3.9, was a bad idea, so we're deprecating it in 3.11 and plan to remove it in 3.13.

Lib/asyncio/futures.py

@@ -8,6 +8,7 @@ import concurrent.futures
 import contextvars
 import logging
 import sys
+import warnings
 from types import GenericAlias
 
 from . import base_futures
@@ -150,6 +151,11 @@ class Future:
         change the future's state to cancelled, schedule the callbacks and
         return True.
         """
+        if msg is not None:
+            warnings.warn("Passing 'msg' argument to Future.cancel() "
+                          "is deprecated since Python 3.11, and "
+                          "scheduled for removal in Python 3.14.",
+                          DeprecationWarning, stacklevel=2)
         self.__log_traceback = False
         if self._state != _PENDING:
             return False