EDIT: This last line of the article pretty much sums it up: "We structured the transition thinking the community would come along with us in leaving Python 2 behind, but that turned out not to be the case and instead we have taken some more time and are using a Python 2/3 compatible subset of the language to manage the transition."
I wonder if python 3.6 or 3.7 will acquiesce to this and give an option for something like "from __past__ import str" like we have for __future__ in 2.7 to make these backward incompatibilities easier to deal with.