This is such a big change and breaks the backward compatiblity, to me it is very similar to what python community has done when moving to python 3.
looking back, 2 to 3 was such a slow (and maybe painful) move due to the incompatibility. but i believe the community has learned so much and i believe this time we should be able to handle this transition much smoothlier.
I would think it is better to kick off python 4 for this pep. we can have python3 and python4 in parallel just like we did for python 2 and 3.
Developers have learned so much from last transition, and i think we will be in a much better shape if we do this again. it is a much cleaner way for doing such things, although it may appear there is more work to be done.