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

[JostMigenda]

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” 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 RSE-Sheffield#75.