Bump furo from 2021.8.31 to 2021.9.22 in /requirements

[dependabot[bot]]

Bumps furo from 2021.8.31 to 2021.9.22.

Changelog

Sourced from furo's changelog.

Changelog

2021.09.22 -- Cavalier Canary

• Restyle API documentation signatures, to have a background and use monospace fonts.
• Reduce spacing for items in API documentation.
• Improve the presentation of links in site-wide announcement.
• Only add a border on code inside paragraphs.
• Use noscript for presenting "search needs JS" message.

2021.09.08 -- Balmy Blue

• Prevent screen-reader-only content from showing up in Sphinx search results.
• Improve support for various footer configurations.

2021.08.31 -- Aspiring Avocado

• First stable release! 🎉
• Document stability policy.
• Tweak API styling selectors.
• Drop reference to no-longer-used pygments_dark.css.
• Eagerly set the light/dark mode theme, when loading a page.

2021.08.17.beta43

• Add support for sphinx-design.
• Document sidebar title customization.
• Don't show "Contents" on pages without h1 headings.
• Add border to inline code, to improve contrast.
• Reduce contrast on dark-mode text.

2021.08.11.beta42

• Fix esoteric failure due to inability to write pygments.css.
• Improve overscroll behaviour.

2021.07.31.beta41

• Adapt for newer sphinx-copybutton design
• Improved screen reader experience
• Bring back asset digests, to avoid caching-related issues

... (truncated)

Commits

• 2ef767d Prepare release: 2021.09.22
• bf56ab4 Update changelog for release
• 2450fed Only put a border on code inside paragraphs
• 6fe884c Add pdoc3 as an inspiration
• 4012c4d Merge pull request #245 from jdknight/use-noscript-for-search-js-detection
• 3c4e948 Tweak the look of API documentation
• 8b83d0c Reduce the spacing for API docs
• 677d421 Add an annoucement to the main site!
• 308381e Make links in announcements nicer to look at
• b5aa88b Add a reference to shoutouts.dev
• Additional commits viewable in compare view

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Superseded by #222.