use sorted(x, key=...) since we shouldn't compare entities

entities.py

@@ -61,7 +61,7 @@ class Email(TreeMixIn, AnyEntity):
                     alternatives.append(set(p.eid for p in part.alternative))
                     alternatives[-1].add(part.eid)
             else:
-                result.append((part.ordernum, part))
+                result.append(part)
         if alternatives:
             for altset in alternatives:
                 selected = [parts_by_eid[peid] for peid in altset
@@ -70,9 +70,8 @@ class Email(TreeMixIn, AnyEntity):
                     selected = selected[0]
                 else:
                     selected = parts_by_eid[altset.pop()]
-                result.append((selected.ordernum, selected))
-        result.sort()
-        return [p[1] for p in result]
+                result.append(selected)
+        return sorted(result, key=lambda x: x.ordernum)
 
     def references(self):
         result = set()