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.

[guitargeek]

Python 2 support has been dropped in ROOT, CMSSW, and practically speaking also in combine with the recent PR that removed the Python 2 environment from the CI:

• Deprecating py2 for combine v10 #1014

So I guess this issue can be closed?

[nsmith-]

Yes py2 is gone. We could in principle also now clean up some of the migration patches. @guitargeek do you happen to remember if python-modernize could be used again to automate some conversions?

[guitargeek]

I found an interesting tool that can be used for that! See the linked PR I just opened.

Does combine have an official "minimum supported Python version"? For ROOT it's Python 3.8, as of ROOT 6.30 by the way. Depending on the minimum version, the pyupgrade tool can also do more aggressive upgrades.