I'm not sure why you say "Python 3-esque str.format" in the description. str.format() was added in Python 2.6. I see that logging support for that style (by using style='{') was added in Python 3.2.
I'm not opposed to the change, as it seems like a slight readability enhancement, although generally I think there's a consensus against using mandating str.format() usage everwhere in Django and prohibiting the %s syntax. I think there was a ticket but I can't find it at the moment.
My only concern with the proposal is whether it could break backwards compatibility. I see an update to ServerFormatter.uses_server_time() was required. I'm not sure if it's likely that user code may make similar assumptions about the format string.