returns lines with max n characters

    :Example:
        >>> print (chunks_str('123456X', 3))
        123
        456
        X

def chunks_str(str, n, separator="\n", fill_blanks_last=True):
    """returns lines with max n characters

    :Example:
        >>> print (chunks_str('123456X', 3))
        123
        456
        X
    """
    return separator.join(chunks(str, n))