D7181: py3: make chistedit render

[Yuya Nishihara]

> +        return b"#%s %s %d:%s   %s" % ((b'%d' % self.origpos).ljust(2),
> +                                       action.ljust(6), r, h, desc)
> +
> +    __bytes__ = __str__

`__str__ = encoding.strmethod(__bytes__)` can be used instead. `__str__`
of Python3 must return unicode string.