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
Why `seq` instead of common commands like `ls` and `git`? `seq` is a useful example. However, it's not a very common command so people reading this might not understand it's a shell command being executed. Why not both examples?