Support python 3.13

[jfrost-mo]

What problem does your feature request solve?

Python 3.13 is now out. We should support it in CSET.

Describe the solution you'd like

This should just need the conda lock file generation updating to also include python 3.13, though testing will be needed to ensure it works.

Describe alternatives you've considered

This isn't super time critical, but would be nice to have.