Migrated to Python 3 and fixed a bug in xgb defs

[kdubovikov]

Hi, I have migrated the codebase to Python 3 and fixed a bug in defs/xgb.py. Hope this will be useful

[zygmuntz]

Thanks, but it won't work with Python 2 now, will it?

[kdubovikov]

Yes, you are right. I will manage this with from __future__ import print_function and update the PR. Then it will work with Python 2 and 3 simultaneously

UPDATE please see new commit, you may check if it works with Python 2 now

[zygmuntz]

You haven't updated all the prints in defs/* and defs_regression/*, though