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