Raise the minimum python version to 3.9

[sveinse]

python-can just raised the minimum python version to 3.9 hardbyte/python-can#1931 . Since canopen is dependent on python-can, I suggest we follow suit. Python 3.8 went EOL in October 2024.

[acolomb]

Let's do that immediately after reaching a v2.4.0 release. I don't think we're going to include changes that require bumping the minimum version before then.