Dropping support for Python 2

[pitkajuh]

On many Python files, there is line

from __future__ import absolute_import, print_function

which is used for compatibility for both Python 2 and Python 3. Root 6.32 dropped support for Python 2. Is there any plans stop supporting Python 2 in this project? I think it has to be done at some point, especially if newer versions of Root will be used as prerequisite.

Migration to Python 3 has been discussed in #759 and #647, but as far as I can tell dropping support for Python 2 has not been discussed.