For the vast majority of projects, upgrading (or working with both PY2 and PY3 at the same time) is not that big of a deal unless the project you're working on is really, really old (2.3 days). At work we run unittests targeting different python versions and the most time I've spent changing a typically 4k loc project's codebase to work on both 2 and 3 has been around 4 hours. The average around 1.