Drop the python2 compatibility

[XVilka]

Python 2.7 will not be maintained past January 1st, 2020.

2 months left until the official EOL. Makes sense to drop the python2 support by that time.

https://docs.python.org/3/howto/pyporting.html

[jbpoittevin]

Compatibility with python 3 is OK, actively breaking python 2 is not a target.
Closing the issue.