Increase default Python version (2025)

[mhsmith]

Python 3.8 will be desupported in 2024-10, so we should change our default Python version as soon as possible. See checklist in target/README.md, and edit it as we go.

But first, we'll have to provide a complete set of packages for that version. Specifically, we should provide all the same packages as we did for 3.8, except for those which are obsolete, or don't support the new Python version yet, or aren't worth the effort (e.g. tensorflow).

Since Python 3.13 will be the first version to have official Android support, our preferred approach would be to help the upstream developers release their own official Python 3.13 package builds on PyPI, and then switch Chaquopy's default directly from 3.8 to 3.13.