Re-introduce iteration over keys

It was removed in changeset 808e05f4898c but indeed we need a copy list of the
keys because the dictionary is modified during the iteration.

trytond/model/workflow.py

@@ -33,7 +33,7 @@ class Workflow(object):
 
                 result = func(cls, filtered, *args, **kwargs)
                 if to_update:
-                    for record in to_update:
+                    for record in to_update.keys():
                         current_state = getattr(record, cls._transition_state)
                         if current_state != to_update[record]:
                             del to_update[record]