I did the work for a reasonably sized project recently - a few hundred thousand LOC. It was long, boring, risky work. Let me rattle off some of the tasks.

Audit all strings coming in and going out for encoding issues. Update all dependencies to their python 3 equivalent. Replace dependencies that hadn’t been updated (typically older django dependencies). Use python-future to bulk update incompatibilities. Changes to metaclasses were annoying. Force all uses of pickle to use protocol version 2. I documented some more during the migration on Twitter https://twitter.com/jarshwah/status/1209381850822496256?s=21

We began getting the code base into a compatible position about 1.5 years earlier. A final push of 3-4 weeks of work got it over the line, with many bug fixes after the deployment.

Other older larger systems will have similar problems at a larger scale.

This isn’t a condemnation by the way. Python 3 is better. The only reason we held out so long was because of the business justification. Once we couldn’t wait any longer it got prioritised.