Bump markdown from 3.8.2 to 3.9

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 3.8.2 to 3.9.
- [Release notes](https://github.com/Python-Markdown/markdown/releases)
- [Changelog](https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md)
- [Commits](Python-Markdown/markdown@3.8.2...3.9.0)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: '3.9'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
-Markdown==3.8.2
+Markdown==3.9
 mike==2.1.3
 mkdocs-git-revision-date-plugin==0.3.2
 mkdocs-macros-plugin==1.3.7