gh-137549: Modify object to BaseExceptionGroup in asyncio.Timeout

Lib/asyncio/timeouts.py

@@ -114,7 +114,7 @@ async def __aexit__(
                     raise TimeoutError from exc_val
                 elif exc_val is not None:
                     self._insert_timeout_error(exc_val)
-                    if isinstance(exc_val, ExceptionGroup):
+                    if isinstance(exc_val, BaseExceptionGroup):
                         for exc in exc_val.exceptions:
                             self._insert_timeout_error(exc)
         elif self._state is _State.ENTERED:

Misc/NEWS.d/next/Library/2025-08-15-14-12-29.gh-issue-137549.2DaXja.rst

@@ -0,0 +1,2 @@
+Modify ``__aexit__`` method of ``asyncio.Timeout`` class to handle the superclass of ExceptionGroup object.
+Patch by gunyu1019.