fix a indices bug for categorical-datetime columns

This is to fix a bug reported in pandas-dev#26859

Author: alexifm

pandas/core/groupby/ops.py

@@ -232,6 +232,7 @@ def indices(self):
             label_list = [ping.labels for ping in self.groupings]
             keys = [com.values_from_object(ping.group_index)
                     for ping in self.groupings]
+            keys = [np.array(key) for key in keys]
             return get_indexer_dict(label_list, keys)
 
     @property