Why didn't they...
Make a simple transition tool. Takes in "python 2(.7)" and emits "python 3". The parser for python2.x already exists, just change what it outputs. Sure it might not be "optimal", but it ought to work. If it doesn't (something that doesn't translate) emit a nice message and a couple of suggestions, and let it go.
Sounds like the way to go for me.
Of course, I prefer nice ANSI C89, which doesn't have such fluff as '//' comments and the like (and MISRA seems to agree!). Of course YMMV!