CI: drop Python 3.9 from the NeoPlexon2Test matrix

[AxelNoun]

plexon2-testing.yml still tests Python 3.9, but neo has required Python >= 3.10 since #1808 (2026-01-29). The 3.9 jobs fail at pip install --upgrade -e . (ERROR: Package 'neo' requires a different Python: 3.9.25 not in '>=3.10', seen in the runs of 2026-10-04 and 2026-07-12) and, with fail-fast: true, cancel the 3.12 jobs.

This PR removes 3.9 from the matrix, leaving 3.12 with numpy 1.26 and 2.0. Replacing 3.9 with 3.10 would instead keep a job on the minimum supported version; I have limited the change to a removal, but I can switch to that if you prefer.

The last successful run is from 2025-12-21. The five runs from 2025-12-28 to 2026-01-25 failed before the Python floor changed, and their logs are no longer available. In the 36 weekly runs since 2026-02-01, the failed job was a 3.9 job, except on 2026-04-05 (3.12 / numpy 1.26). This change lets the 3.12 jobs run again; it does not guarantee that they pass.

Not tested in CI: the workflow only runs on schedule or workflow_dispatch, so this PR does not trigger it. The edited file loads with yaml.safe_load and the matrix expands to two jobs.

[zm711]

I'm fine with this because we are not currently testing plexon as it is designed for Windows and we have had long term issues with getting it to run consistently on Linux runners. But we should keep it up to date for when we get the runner issue solved. I just want to make sure the core tests pass and then I'll merge this one.