Make sure to iterate over a copy of the public_keys dictionary

util/security/instancekeys.py

@@ -26,7 +26,7 @@ class InstanceKeys(object):
       keys[key.kid] = ExpiresEntry(key, key.expiration_date)
 
     # Remove any expired or deleted keys from the public keys cache.
-    for key in self.public_keys:
+    for key in dict(self.public_keys):
       if key not in keys:
         self.public_keys.pop(key)