🩹 Fix repr issue with numpy>2 str values

This caused the following test error:
```py
FAILED glotaran/utils/test/test_io.py::test_create_clp_guide_dataset_errors - assert "Unknown clp_...'species_3')]" == "Unknown clp_... 'species_3']"

  Skipping 49 identical leading characters in diff, use -v to show
    ls are:
  -  ['species_1', 'species_2', 'species_3']
  +  [np.str_('species_1'), np.str_('species_2'), np.str_('species_3')]
```

glotaran/utils/io.py

@@ -442,7 +442,7 @@ def create_clp_guide_dataset(
     if clp_label not in dataset.clp_label:
         raise ValueError(
             f"Unknown clp_label {clp_label!r}. "
-            f"Known clp_labels are:\n {list(dataset.clp_label.values)}"
+            f"Known clp_labels are:\n {[str(label) for label in dataset.clp_label.values]}"
         )
     if "model_dimension" not in dataset.attrs:
         raise ValueError(