book: update-theme.py needs updating

[Kijewski]

mdBook changed their template to render the sidebar without JS:

• Load the sidebar toc from a shared JS file or iframe rust-lang/mdBook#2414

This change breaks our update-theme.py script, which edits the sidebar template. Updating the script should not be too difficult, because the new template structure should be easier to parse than before.

In any case, this problem should to considered a breaking bug before we release the next version. If we haven't updated the script until then, we could pin the requirement to the last version before rust-lang/mdBook#2414 was merged.