So, as with Jesse's example, what I'm wondering is whether or not 2to3 will fix that for you (or even flag it). The basic difference between the two alternatives I suggested is whether we force people to put their python function through that converter before we'll even try to run it. Subtleties that 2to3 doesn't catch seem like non-reasons to insist on applying it.
The 2018 vintage of 2to3 didn't catch it.
It's not firsthand knowledge, but I just watched a nearby team have some production issues where one library couldn't fetch b'http://foo.org' so I'm guessing 2to3 still doesn't catch those things, or they stopped using it.