docs: def unpack_epub(file, directory):
      """Unpack the given ``file`` (a file-path or file-like object)
      to the given ``directory``.
      """
      if zipfile.is_zipfile(file):
          # Extract the epub to the current working directory.
          with zipfile.ZipFile(file, 'r') as zf:
              zf.extractall(path=directory)