Remove references to Python 2 in preparation for the release of Root 6.32

[pitkajuh]

Support for Python 2 will be dropped in Root version 6.32. Please see https://github.com/root-project/root/issues/13861. The purpose of this PR is to make updating Root easier.

[guitargeek]

Hi! I have committed already parts of it. For the rest of it, can you please open a PR in the main ROOT repo, changing this subdirectory? https://github.com/root-project/root/tree/master/interpreter/cling

This repo is just a mirror of that directory. Thanks!

[ferdymercury]

Moved to https://github.com/root-project/root/pull/16098