Minor tweak to PyConf JSON encoder to support set properties

PyConf/python/PyConf/utilities.py

@@ -101,10 +101,10 @@ def _read_options_pkl_impl(filename, all_opts=True):
 
 class _PropEncoder(json.JSONEncoder):
     def default(self, obj):
-        try:
-            obj = obj.toStringProperty()
-        except AttributeError:
-            pass
+        if isinstance(obj, set):
+            return sorted(obj)
+        elif hasattr(obj, "toStringProperty"):
+            return obj.toStringProperty()
         return obj