#3720: Interpreter crashes when an evil iterator removes its own next function.

Now the slot is filled with a function that always raises.

Will not backport: extensions compiled with 2.6.x would not run on 2.6.0.
diff --git a/Misc/NEWS b/Misc/NEWS
index 6a6f1e5..e39fe8a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #3720: Fix a crash when an iterator modifies its class and removes its
+  __next__ method.
+
 - Issue #4893: Use NT threading on CE.
 
 - Issue #4915: Port sysmodule to Windows CE.