Drop Python 3.9 support

[fedebotu]

Happy New Year, everyone!

Time for some improvement~ Following #236 from @hyeok9855 , I realized that keeping support for Python 3.9 may be annoying. Several recent libraries support from Python 3.10 onwards, such as PyVRP (see here) and the GFlowNet library (see here). Also, with Python >= 3.10 we will be able to:

• replace Optional[int] by int | None
• replace Union[int, list] by int | list
• use match/case

What do you think @cbhua @LTluttmann ? This would also fix our issues in the GFACS @hyeok9855

[cbhua]

Happy new year mates! Time to back to work.

Based on the status of the Python version, Python 3.9 will end the supporting on 2025.10. Even though I think there are quite a few users who still hold Python 3.9, RL4CO is a package with up-to-date model/environment features for mostly research (in the current state). Also based on my experience, from Python 3.10, the compatibility of Python to the previous version has been improved a lot. Generally for me it would be fine to end the supporting for Python 3.9 at the current time.

[fedebotu]

Happy new year mates! Time to back to work.

Based on the status of the Python version, Python 3.9 will end the supporting on 2025.10. Even though I think there are quite a few users who still hold Python 3.9, RL4CO is a package with up-to-date model/environment features for mostly research (in the current state). Also based on my experience, from Python 3.10, the compatibility of Python to the previous version has been improved a lot. Generally for me it would be fine to end the supporting for Python 3.9 at the current time.

Great! Exactly, since we are in the research domain, I think it is safe to assume upgrading Python versions wouldn't be an issue. According to pypistats the number of installs (at least directly from PyPi) for 3.9 has been pretty low:

[cbhua]

Wow, this is a really cool statistic plot! Love it. Then it wouldn't hurt that much to drop the support of Python 3.9. But let's write a notification in the Slack channel 🙌