Remove support for Python 2 switch to Python 3

It's year 2019 and time to remove support of the deprecated technology [Python 2](https://pythonclock.org/). With current knowledge, the maintenance of Python 2 will be discontinued in January 2020:

- https://github.com/python/devguide/pull/344
- https://www.python.org/dev/peps/pep-0373/