docs: def to_unicode(value):
    """Returns a unicode string from a string, using UTF-8 to decode if needed.
  
    This function comes from `Tornado`_.
  
    :param value:
      A unicode or string to be decoded.
    :returns:
      The decoded string.
    """
    if isinstance(value, str):
      return value.decode('utf-8')
  
    assert isinstance(value, unicode)
    return value