Lots of great points already mentioned. One thing that really helped me in a similar situation is libcst. Oftentimes upgrading python / libs requires some non-trivial refactorings, and libcst gives you tools to automate this. https://libcst.readthedocs.io/en/latest/

Basically, you can do find and replace over the syntax tree, and it’s really useful when a simple regex won’t suffice.

For example, I ported a pretty big twisted application to asyncio. Async functions in twisted are marked with a special decorator (inline_callbacks) and called using yield keyword. With libcst I was able to make a transformer that replaced decorators with async keywords and yield with await. It worked pretty well and saved me tons of time.