[Mobile-sig] Python 3 port of DUCK

[David Boddie]

Earlier this year someone asked me nicely about running DUCK using Python 3,
and offered to look into porting the code since I've been busy with work.

Anyway, I found/made some spare time to start a quick and dirty port and,
after some effort, put the end result in a branch on GitLab:

  https://gitlab.com/dboddie/DUCK/tree/python3

I built and ran the suite of test applications for the Serpentine compiler,
and they all pass, so it's probably usable on a basic level.

One of the main changes is that, because the compiler uses the built-in
Python parser, some programs needed updating to Python 3-compliant syntax.
I imagine that I'm the only person who is affected by that, so maybe it's not
the end of the world.

I hope it's useful for someone - other than me, of course. ;-)

David