Fix storing of optgroups in Choices._triples

In particular, when an optgroup is passed as a double (pair) to the
`Choices` constructor, it should be converted to a triple before being
stored in `_triples`.

Author: mthuurne

model_utils/choices.py

@@ -103,7 +103,7 @@ def _process(
                         group_name = choice[0]
                         subchoices = choice[1]
                         tc: list[_Triple] = []
-                        triple_collector.append((group_name, tc))
+                        triple_collector.append((group_name, group_name, tc))
                         dc: list[_Double] = []
                         double_collector.append((group_name, dc))
                         self._process(subchoices, tc, dc)