deps: update sphinx to 6.0.1

[onerandomusername]

Summary

update sphinx to 6.0.1. This is semi-untested, making a pr for the rtd preview.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running task lint
  • I have type-checked the code by running task pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[shiftinv]

• favicon in templates was replaced with favicon_url in 6.0 (also the shortcut rel is obsolete: Remove obsolete HTML keyword link rel="shortcut" sphinx-doc/sphinx#11069)
• towncrier 22.12 supports orphan fragments (i.e. no linked issues), our template should take that into account as it outputs - Changed xyz. () otherwise
• searchindex.js is empty, for some reason

[shiftinv]

Update, search issue seems to be this one: sphinx-doc/sphinx#11163. We should pin sphinx==6.0.1 or wait for an update (don't think there's an eta yet).