#11767: use context manager to close file in __getitem__ to prevent FD leak

All of the other methods in mailbox that create message objects take care to
close the file descriptors they use, so it seems to make sense to have
__getitem__ do so as well.

Patch by Filip GruszczyƄski.
diff --git a/Misc/NEWS b/Misc/NEWS
index 6cc0f03..ec8b72e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -25,6 +25,8 @@
 Library
 -------
 
+- Issue #11767: Correct file descriptor leak in mailbox's __getitem__ method.
+
 - Issue #12133: AbstractHTTPHandler.do_open() of urllib.request closes the HTTP
   connection if its getresponse() method fails with a socket error. Patch
   written by Ezio Melotti.