#15040: Close files in mailbox tests for PyPy compatibility

Original patch by Matti Picus.
diff --git a/Misc/ACKS b/Misc/ACKS
index 958951f..88b7ed0 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -762,6 +762,7 @@
 Adrian Phillips
 Christopher J. Phoenix
 Neale Pickett
+Matti Picus
 Jim St. Pierre
 Dan Pierson
 Martijn Pieters