Can't talk for Pythran devs and others, but in the case of shedskin, one of the issues is the removal of the “compiler” module in Python 3, on which shedskin relies to, well, parse the Python code. This, plus some syntax changes, that are not really bigger that some that were introduced between, say, 2.6 and 2.7, but that are mandatory and breaking backward compatibility.

So, nothing “hard”, really, you just have to do the work again (and prepare to support two versions for a long time, instead of virtually only one).