Move code to Python 3
Python 2.x End of life has been set to 2020 (https://www.python.org/dev/peps/pep-0373/).
Current blockers: pdfminer (Dev has no intention to move to P3K: https://github.com/euske/pdfminer/pull/71). pdfminer.six fork is not active anymore (https://github.com/euske/pdfminer/pull/71#issuecomment-246118657)