docs: def filtered_notebook_metadata(notebook):
      """Notebook metadata, filtered for metadata added by Jupytext itself"""
      metadata = copy(notebook.metadata)
      metadata = filter_metadata(metadata,
                                 notebook.metadata.get('jupytext', {}).get('notebook_metadata_filter'),
                                 _DEFAULT_NOTEBOOK_METADATA)
      if 'jupytext' in metadata:
          del metadata['jupytext']
      return metadata