        >>> u.setconfig(s, b'val2', b'42.5 kb')
        >>> u.configbytes(s, b'val2')
        43520
        >>> u.configbytes(s, b'unknown', b'7 MB')
        7340032
        >>> u.setconfig(s, b'invalid', b'somevalue')
        >>> u.configbytes(s, b'invalid')
        Traceback (most recent call last):
            ...
        ConfigError: foo.invalid is not a byte quantity ('somevalue')
        r