add support for python 3

[DrSensor]

This convert roshelper to support both python2 and python3 using this command:

$ futurize -0 -w -a -n **.py
$ futurize -0 -w -a -n roshelper/**.py
$ futurize -0 -w -a -n roshelper/**/*.py

I also changes the absolute import to relative import so anyone can try the latest version by:

pip3 install https://github.com/wallarelvo/roshelper.git