Drop Python 3.9 support in favor of 3.12 and 3.13

[thalassemia]

I was able to update requirements.txt to work on Python 3.10-3.13. Unfortunately, the latest possible version of networkx that installs on Python 3.9 is 3.2.1, and a change in networkx 3.4 breaks our plot_topology function. I could add logic to determine the version of networkx and do different things depending on whether it is greater or less than 3.4, but that sounds unnecessarily complicated. Instead, I think it makes more sense to just bump the required Python version to 3.10 and networkx version to 3.4. For context, Python 3.9 reaches end-of-life in 2025-10, and big packages like Numpy and Scipy have already dropped support for it.

Other changes include:

• Assorted pylint and mypy fixes
• Change default multiprocessing start method to forkserver on Linux to fix python/cpython#110770 (default will be forkserver in Python 3.14 anyways)

---

By creating this pull request, I agree to the Contributor License Agreement, which is available in CLA.md at the top level of this repository.

[thalassemia]

Thanks @thalassemia! So is there topology graphing as it currently stands?

plot_topology works in the current release as long as you have networkx<3.4. I think those older versions of networkx should work on Python 3.10-3.13 as well. I don't think it is a good idea to cap the networkx version to address the issue with networkx 3.4+.

[eagmon]

@thalassemia -- looks like there are still some 3.9-related tests hanging. Can you remove these from the GitHub actions?

[thalassemia]

I think those are required by some repository setting that I can't access. Instead of hanging they're just not being run. These docs might be helpful.