[keyneus]

Type annotations. They're the sole reason we moved from Python2 to Python3, and it was completely worth it. With mypy, we now have at least some semblance of a type system and type checking, which is a huge win.

Unicode support is vastly improved.

Finally there are have specific exceptions like FileNotFoundError so you don't have to catch OSError and then check errno. I got so tired of that.

No more old-style classes, which means you know you can use super(), which is now less annoying since you don't need to “remind” Python which class you're in.

Lots of other small changes, most of which do lead to improvements, however minuscule. I would argue that unless you are must use a Python2-only library, just go with 3.