bpo-39728: Enum: fix duplicate `ValueError` (GH-22277)

fix default `_missing_` to return `None` instead of raising a `ValueError`
Co-authored-by: Andrey Darascheka <andrei.daraschenka@leverx.com>
diff --git a/Misc/ACKS b/Misc/ACKS
index 8b0d7a4..2628d6b 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -433,6 +433,7 @@
 Dima Dorfman
 Yves Dorfsman
 Michael Dorman
+Andrey Doroschenko
 Steve Dower
 Allen Downey
 Cesar Douady