There won't be breaking changes from 3.3->3.5, so the code changes are the same either way. I guess skipping a release saves the effort of making the release/prioritizing things that appeared in 3.4.
You can implement Your Own languages on pypy so make it 4 :)
Breaking changes I would include would be : remove all the deprecated bits of subprocess or replace it entirely, remove the zip, ftp and other modules in favour of virtual filesystems with a consistent api and finally try and make all the APIs symmetrical (e.g. open always paired with close, push with pop etc).
