Python 3 compatibility

[kubaello]

I tried running the bot with python 3 and fixed errors which I encountered. It seems to be working with both python 2 and 3 now. The changes are:

• use range from six (a python 2/3 compatibility package), which is mapped to xrange in python 2 and range in python 3,
• use integer division operator in calc_axis.py where division resulted in a float in python 3, but an int was expected,
• convert result of download_canvas from a string to a bytearray (unicode strings caused problems in python 3),
• use explicit relative imports when importing anything from src.