POSIX APIs take bytes, generally. Python wraps these APIs to take unicode and doesn't allow you to pass bytes, even if you need to. Filenames, for example, are just bytes, and if you force them to always be valid unicode you will make it so that you can't interact with files that have names that aren't valid unicode. That's just one example.
An extremely frustrating part of the Python 3 migration is how many times Python module maintainers have had to hear "oh, now it's safe to migrate." This page currently leads off with a comment saying it's been fine any time since 3.4. You say 3.6. When I was maintaining a popular Python module, I heard the same at 3.1, and 3.2. (I didn't maintain it long after that.)
