I had issues which I initially believed were related to python 3 but may have been XCode issues - I got it to work by doing all of these things but I'm not sure what was actually necessary - so I'll put all the steps in that I can recall.
Ultimately, I was seeing python stack traces when running tools/dev/v8gen.py x64.optdebug
; the resulting stack trace looked like this:
tools/dev/v8gen.py x64.optdebug