[PEP-654] add None check in subgroup example (GH-2132)

Joe Gottman alerted us to an error in this example, which is missing a None check.

Author: iritkatriel

pep-0654.rst

@@ -1023,7 +1023,9 @@ OS errors, while letting all other exceptions propagate.
    try:
        low_level_os_operation()
    except* OSError as errors:
-       raise errors.subgroup(lambda e: e.errno != errno.EPIPE) from None
+       exc = errors.subgroup(lambda e: e.errno != errno.EPIPE)
+       if exc is not None:
+           raise exc from None
 
 
 Caught Exception Objects
@@ -1478,7 +1480,7 @@ ways, both the design and the exposition. For this we appreciate all those who
 contributed ideas and asked good questions:  Ammar Askar, Matthew Barnett,
 Ran Benita, Emily Bowman, Brandt Bucher, Joao Bueno, Baptiste Carvello,
 Rob Cliffe, Nick Coghlan, Steven D'Aprano, Caleb Donovick, Steve Dower,
-Greg Ewing, Ethan Furman, Pablo Salgado, Jonathan Goble, Thomas Grainger,
+Greg Ewing, Ethan Furman, Pablo Salgado, Jonathan Goble, Joe Gottman, Thomas Grainger,
 Larry Hastings, Zac Hatfield-Dodds, Chris Jerdonek, Jim Jewett, Sven Kunze,
 Łukasz Langa, Glenn Linderman, Paul Moore, Antoine Pitrou, Ivan Pozdeev,
 Patrick Reader, Terry Reedy, Sascha Schlemmer, Barry Scott, Mark Shannon,