Fix py3 iterator behavior.

This was already committed in another branch, but we have so many flying
around now I can't find it.  Credit due originally to Adam Coddington, I
believe.

taskw/warrior.py

@@ -97,7 +97,7 @@ def _extract_annotations_from_task(self, task):
                 else:
                     annotations.append(v)
 
-        for key in task.keys():
+        for key in list(task.keys()):
             if key.startswith('annotation_'):
                 annotations.append(task[key])
                 del(task[key])