In general the approach is pretty heavily dependent on a trustworthy test suite. If you don't have that, you'll have to write a lot of tests which will take more than 40 hours, but... Python is pretty heavily dependent on having a trustworthy test suite.

All of the codebases I migrated were Django codebases.

Basically, I would:

1. Upgrade to the latest 2.x, run the test suite and a bunch of manual tests, and fix any errors/warnings. 2. Upgrade all the dependencies one by one to versions that support version 3, run the test suite and the manual tests, and fix any errors/warnings. This was generally the hardest part, because some projects had imported a lot of bleeding edge nonsense which was abandoned before Python 3 support was implemented, so I had to a) encapsulate the dependency, and then b) rewrite the encapsulated functionality either with my own code, or with a more up-to-date library. This is a predictable result of importing nonsense, and I blame the developers who did that, not the Python team. 3. Upgrade to 3.x, run the test suite of manual tests, and fix any errors/warnings.