Check a string is utf-8 encoded

    :param bytes txt: utf-8 string
    :return: Whether the string\
    is utf-8 encoded or not
    :rtype: bool

def _is_utf_8(txt):
    """
    Check a string is utf-8 encoded

    :param bytes txt: utf-8 string
    :return: Whether the string\
    is utf-8 encoded or not
    :rtype: bool
    """
    assert isinstance(txt, six.binary_type)

    try:
        _ = six.text_type(txt, 'utf-8')
    except (TypeError, UnicodeEncodeError):
        return False
    else:
        return True