python3 support

This is based on my update-ci branch.

See the diff here:
https://github.com/bukzor/python-clom/compare/bukzor:buck-ci-update...bukzor:buck-py3
Ah, I see. Didn't know you had them under test. They'll work for now. I'll think of some examples that don't depend on the environment but illustrate more common commands.

I'm only online for the next 30 mins or so today. I'll finish this PR.