in ftp cache pruning, avoid changing the size of a dict while iterating over it (closes #21463)

Patch by Skyler Leigh Amador.
diff --git a/Misc/ACKS b/Misc/ACKS
index 571f1a6..344a29b 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -32,6 +32,7 @@
 Ray Allen
 Billy G. Allie
 Kevin Altis
+Skyler Leigh Amador
 Joe Amenta
 A. Amoroso
 Mark Anacker
diff --git a/Misc/NEWS b/Misc/NEWS
index 8f05a13..f581cd4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,8 @@
 Library
 -------
 
+- Issue #21463: In urllib.request, fix pruning of the FTP cache.
+
 - Issue #21618: The subprocess module could fail to close open fds that were
   inherited by the calling process and already higher than POSIX resource
   limits would otherwise allow.  On systems with a functioning /proc/self/fd