switchers.js not compatible with Sphinx 6 / requires jQuery

[hugovk]

1. Open https://docs.python.org/3.12/ and check the language and version switchers are shown in the header:

2. Check the footer: "Created using Sphinx 4.5.0."
3. Open https://docs.python.org/3.13/ and see the switchers are not shown:

4. Check the footer: Created using Sphinx 6.2.0."
5. Check the browser console:

switchers.js:168 Uncaught ReferenceError: $ is not defined
    at switchers.js:168:3
    at switchers.js:182:3

This is because Sphinx 6 removed jQuery:

#7405: Removed the jQuery and underscore.js JavaScript frameworks.

These frameworks are no longer be automatically injected into themes from Sphinx 6.0. If you develop a theme or extension that uses the jQuery, $, or $u global objects, you need to update your JavaScript to modern standards, or use the mitigation below.

https://www.sphinx-doc.org/en/master/changes.html#release-6-0-0-released-dec-29-2022 has a couple of options on how to re-add jQuery, but it would be better rewrite https://github.com/python/docsbuild-scripts/blob/main/templates/switchers.js to use vanilla JavaScript.