Python3 traceback updates

I've changed the `sitecustomize.py` file to work with tracebacks on my local Python 3 setup. There were previous issues that mentioned fixing this but the most recent python.tmbundle did not work for me. Hope that this gives some ideas on how to handle Py3 support. It works but I do have time to fully validate the solution (e.g. check Py2 works properly and check if there is a better way to check for Py3 other than a failed import).