One main reason in my opinion as to why Python 2.7 stuck around for so long was that everyone agreed that it was the last stable version of Python 2, and the devs could make Python 3 changes without worrying significantly about backward compatibility. I've been using Python3 since Python 3.3 but have had to write code that was Python 2 compatible for large code bases for about 5 years because I knew colleagues / users of my packages would not upgrade. This meant using a select subset of features in Python 3 that had been backported to a version of Python 2.7 that I knew I could get my colleagues / users to upgrade to. It has been great to watch the language evolve and Python 3 really gets a lot of things right, but adding breaking syntax features every minor release is extremely annoying. I have to have a mental checklist of all the different possible subsets of features I have access to given a minimum version of Python I want to support. I've spent the last couple of years in my professional career converting large Python 2 only code bases to Python 3.3+. But if someone wants to use async, I need to bump the minor version. If someone wants to use type hinting I have to bump the minor version. If someone wants to use f-strings I have to bump the minor version. If someone wants to use data classes I have to bump the minor version. It's nuts (to me anyway)!
This sounds rant-y but I genuinely want to know what other people are doing to mitigate this. I love Python! I have spent years advocating its merits. But thinking about large codebases in Python just worries me and the future doesn't look promising here. Are Python developers hoping that once 2020 arrives they'll have a stable Python release and work on Python 4. We'll have the Python 2/3 split all over again!
My personal opinion is that Python needs an officially maintained translator for the latest Python features back to older versions. My current favorite package is py-backwards [0] but it is rather unmaintained at the moment.