Bump furo from 2021.7.31b41 to 2021.8.31 in /requirements

[dependabot[bot]]

Bumps furo from 2021.7.31b41 to 2021.8.31.

Changelog

Sourced from furo's changelog.

Changelog

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

2021.07.28.beta40

• Add spacing around light theme / dark theme / auto theme toggle.

2021.07.28.beta39

• Site visitors can now force light theme / dark theme, independently of browser settings. 🎉
• Rework handling of dark theme code block highlighting.
• "Hide Search Matches" shows up in the sidebar, when the user has search matches highlighted.
• Search term highlights are only shown in the page content.
• Fix styling for default aligned tables.
• Enable smooth scrolling.

... (truncated)

Commits

• 4a0f23f Prepare release: 2021.08.31
• 94e5d7b Update changelog for first stable release!
• d96e032 Bump to matching version of black
• 3cd3013 Rework versioning scheme, to release non-beta
• cc7db14 Update changelog
• 4212282 Handle theme colour logic as soon as possible
• ceaa691 Simplify API "property" highlighting selector
• 72b0773 Rewrite API documentation example
• d0d694d Update stability policy reasons + fix typo
• e1e67e6 Call it a stability policy, and exempt betas
• Additional commits viewable in compare view

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