After looking for some time through the changes, I tend to think that the change is good as it is now.

I think it is okay to use either str.format(), either f"str".
Mostly I prefer the second option but sometimes it'll be more readable to specify the first one, i.e.:

        return '<{}.{} instance at {}: {}>'.format(
            self.__class__.__module__,
            self.__class__.__name__,
            hex(id(self)),
            self._data
        )

So, I give my ack for the change as it's an obvious improvement and we should really get rid of the old string formatting '%'.

But as always, we need a second ack here. :) Could you please double-check me, guys?
@tiran @encukou

Looks good to me. Thank you!