Fix pixi pulling newer Python version

[ipmb]

Pixi seems to be jumping to Python 3.14 which is causing a failure in the build https://github.com/lincolnloop/python-package-manager-shootout/actions/runs/18377218212/job/52354121410

[ipmb]

The tests pass with this, but I don't think they are correct. Cold install doesn't seem to be doing anything https://github.com/lincolnloop/python-package-manager-shootout/actions/runs/18383090552/job/52374495457

[edmorley]

For the benchmarks to be comparable, it seems all package managers should run using the same Python version? (And ideally the newest Python version that supports all of the benchmark's dependencies)

Currently I see some inconsistent Python versions, eg:

• Poetry using 3.10 here: https://github.com/lincolnloop/python-package-manager-shootout/actions/runs/18377218212/job/52354121466#step:5:17
• uv using Python 3.11 here: https://github.com/lincolnloop/python-package-manager-shootout/actions/runs/18377218212/job/52354121361#step:6:14

[ipmb]

Yes, the intent is that they're all on the same Python version. Not sure why Poetry is pulling 3.10. Would be happy for a PR to get that on 3.11 like the rest.

Also happy for a PR that bumps to a newer Python version. I'm not sure what the max supported by these dependencies is. That may be why we're on 3.11 :)