ROOT does not configure anymore with Python 3.9

[hahnjo]

Commit f7be28f raised the minimum Python version to 3.10. This is inconvenient because it is the default version on EL9 (I know that it's possible to get newer versions, but that's not the same as "default"). I'm aware that Python 3.9 is declared end-of-life upstream, but EL9 is a very wide-spread distribution in our field. I would therefore like to suggest that we revert that bump as soon as possible. We should also revert root-project/root-ci-images#90 which turned our CI image into a configuration that may not match what users actually have installed.

[guitargeek]

Thanks for opening a discussion about this. It's true that it was a decision not without downsides.

But personally, I would not rush to support Python 3.9 again. Supporting a Python version for 10 years to match the RHEL 9 lifetime is unrealistic, and we should not commit to do so without an explicit experiment or user request. And I'm not expecting any requests to come in, as most experiments don't use the system Python version anyway.

And if you want to use ROOT with Python 3.9 on RHEL 9, you can use ROOT 6.36 - which we still support!

About root-project/root-ci-images#90: I followed the precedent of what was done initially for the Alma 8 image, where the default Python is 3.6 but we installed 3.8: root-project/root-ci-images@7a447d2.

Where I'd agree with you is that it's not optimal that we are not testing the old release branches on all the Python versions that they claim to support. I can investigate how to install multiple Python versions in the same container, so we can pick the appropriate one for each release branch in the CI.