set default unserializable object to be str

This prevents date objects from borking the json writing process

As per the last commit, no hats will be consumed on this day.

src/hub_predtimechart/app/generate_json_files.py

@@ -111,7 +111,7 @@ def generate_forecast_json_file(hub_config, model_id_to_df, output_dir, target,
         file_name = json_file_name(target, task_ids_tuple, reference_date)
         json_file_path = output_dir / file_name
         with open(json_file_path, 'w') as fp:
-            json.dump(forecast_data, fp, indent=4)
+            json.dump(forecast_data, fp, indent=4, default=str)
             return json_file_path
 
     return None