Issue #21905: Avoid RuntimeError in pickle.whichmodule() when sys.modules is mutated while iterating.

Patch by Olivier Grisel.
diff --git a/Misc/ACKS b/Misc/ACKS
index c91d643..3095047 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -494,6 +494,7 @@
 Grant Griffin
 Andrea Griffini
 Duncan Grisby
+Olivier Grisel
 Fabian Groffen
 Eric Groo
 Dag Gruneau
diff --git a/Misc/NEWS b/Misc/NEWS
index 9915e4e..6112a00 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,9 @@
 Library
 -------
 
+- Issue #21905: Avoid RuntimeError in pickle.whichmodule() when sys.modules
+  is mutated while iterating.  Patch by Olivier Grisel.
+
 - Issue #22219: The zipfile module CLI now adds entries for directories
   (including empty directories) in ZIP file.