Bump furo from 2021.8.31 to 2021.10.9 (#412)

Bumps [furo](https://github.com/pradyunsg/furo) from 2021.8.31 to 2021.10.9.
- [Release notes](https://github.com/pradyunsg/furo/releases)
- [Changelog](https://github.com/pradyunsg/furo/blob/main/docs/changelog.md)
- [Commits](pradyunsg/furo@2021.08.31...2021.10.09)

---
updated-dependencies:
- dependency-name: furo
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -12,7 +12,7 @@ sphinx @ https://github.com/sphinx-doc/sphinx/archive/03bf83365eb7a5180e93a3ccd5
 sphinx==4.1.2; python_version < '3.10'
 sphinx-autobuild==2021.3.14
 myst-parser==0.15.2
-furo==2021.8.31
+furo==2021.10.9
 ghp-import==2.0.2
 # myst-parser + docutils==0.17 has a bug: https://github.com/executablebooks/MyST-Parser/issues/343
 docutils==0.17.1