I think the biggest issue seems to be that you need to migrate the whole thing at once. If it could be done incrementally it would be easier.
Although there are ways, you can still incrementally adapt code base to work on both pythons. Also pylint with py3k option, mypy can help. There's also six packages, but many people seem to had good luck with futurize.
There's also something that I tried a while ago and it surprisingly worked (although it might not work that well on larger codebase?), basically you can use Cython (not to be confused with CPython) to compile Python 2 code and then include it in python 3, this would enable migration file by file.
Although there are ways, you can still incrementally adapt code base to work on both pythons. Also pylint with py3k option, mypy can help. There's also six packages, but many people seem to had good luck with futurize.
There's also something that I tried a while ago and it surprisingly worked (although it might not work that well on larger codebase?), basically you can use Cython (not to be confused with CPython) to compile Python 2 code and then include it in python 3, this would enable migration file by file.