> With Python 2 -> 3, my understanding was that everything had to be updated recursively.

You can mix "Python 2" and "Python 3" code, but the problem is (was) that the semantics in Python 3 surrounding string handling changed so much that in reality this was tricky to accomplish, and Python's dynamic untyped nature didn't help either.

But it was definitely possible to write libraries that worked with Python 2 and 3; I've made a few. It was pretty tricky though, and often led to bugs.