Converted to syntax usable with python3

[isaac-ped]

Simply ran python's 2to3 tool on the contents of the repo and added python_requires='>=3' to setup.py.

Brief usage tests showed that it seemed to function fine with python 3 now

[isaac-ped]

I know this isn't maintained, but I wanted to use it so ran 2to3 locally and figured I'd put out a PR just in case.

Fixes https://github.com/agermanidis/autosub/issues/186