Brett Cannon: "Would it make sense to tag the type of the constant in the node somehow?"
It's easy to get the type of a constant: type(node.value). I like using isinstance().
Brett Cannon: "We also make no backwards-compatibility guarantees about the AST, so if it simplifies things to switch entirely to Constant from Num, etc. then I said do it."
Oh, I forgot an important point from the PEP: "[ast.Constant] does not contain line number and column offset informations on tuple or frozenset items." I don't know if it's an issue or not.
I prefer to move step by step. As I wrote, we can decide to use directly ast.Constant later, even before the Python 3.6 release.