It looks like just removing the `:s` is the way to go...

It couldn't find anything about this change in the Python 3.4 release notes, but I guess what they are getting at here is that `{0!s}` (and simply `{0}` already does `str(x)` on the object, so it's redundant to have `{0:s}` also do that, so instead that operates as a sort of type check I guess.