Document the process for upgrading the course's recommended Python version

This should be a checklist (maybe as an issue template?) of steps to go through to verify that the content remains accurate for major CPython releases.

- [ ] Read through “[What’s New in Python 3.x](https://docs.python.org/3/whatsnew/)” document to look for major changes that require changes to the content of the course (e.g. new features like free-threading, JIT, …)
- [ ] Set up new blank environment and install all dependencies from scratch.
- [ ] Run through code snippets and make sure they run without deprecation warnings, etc.
- [ ] Verify that performance claims remain _qualitatively_ correct. (Some quantitative variations between different machines or CPython versions are expected.)
- [ ] Check that dependencies (e.g. NumPy, line_profiler, …) have published wheels supporting the new release.

Major new releases of other dependencies should only affect individual lessons and will usually be caught when checking for deprecation warnings above; so it’s probably sufficient to handle these _ad hoc_, without creating specific checklists for each.

See also previous discussion in https://github.com/RSE-Sheffield/pando-python/issues/75.

No target file or test is named; first review the checklist in this issue and the previous discussion in issue #75, then locate the project’s documentation or issue-template location. Done means a reusable checklist covers CPython release changes, fresh dependency installation, warning-free examples, qualitative performance claims, and dependency wheel support.

Written by the indexing model from the issue text.