Bump mkdocs-material from 8.3.6 to 8.3.9 (#504)

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 8.3.6 to 8.3.9.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@8.3.6...8.3.9)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

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

requirements.txt

@@ -1,2 +1,2 @@
 mkdocs==1.3.0
-mkdocs-material==8.3.6
+mkdocs-material==8.3.9