Feature flags or at worse, transpile. Perl did the first (in form of just writing what version features you want to use in header) and they also went thru "UTF8ication" of the language.
Even if you don't get all old code to work, having say 80% of it working means to start using Py3 you'd need to only initally rewrite 20% of the codebase, run tests and off you go, then migrate rest slowly over time.
Even if you don't get all old code to work, having say 80% of it working means to start using Py3 you'd need to only initally rewrite 20% of the codebase, run tests and off you go, then migrate rest slowly over time.