I used to write a lot of python, and the hard part is that as you write code you're pulling in more and more external libraries, and sooner or later one of them

I used to write a lot of python, and the hard part is that as you write code you're pulling in more and more external libraries, and sooner or later one of them will be incompatible with python 3. Then if you want to continue using python 3 in your own code, you need to spend the effort to port that library to python 3.