Looks like a lot of the guide assumes you want to run on Py2 and 3 concurrently. The time for that has passed to be honest. A clean port is easier to do.
For larger code bases making direct jump straight to Py3 that breaks Py2 compatibility without a transitional period can be problematic especially if it's a library.
The time may have passed, but lots of code is still out there that needs to be updated.
