Port to Python3

[kiwih]

I have done a first pass attempt to convert the codebase in this repository from the now-retired Python 2 into Python3. Unfortunately I'm quite new to this tool and I'm not sure what an appropriate way to test it fully would be - there doesn't appear to be any included automated tests. I analyzed the included TE.PRG and it does this successfully and is then able to produce the graph.

If you can let me know ways to further test the codebase to strengthen this Pull Request please let me know.