Bump markdown from 3.3.7 to 3.5.1

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 3.3.7 to 3.5.1.
- [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@3.3.7...3.5.1)

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

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

requirements.txt

@@ -6,7 +6,7 @@ joblib==1.3.2
 jsmin==3.0.1
 livereload==2.6.3
 lunr==0.7.0.post1
-Markdown==3.3.7
+Markdown==3.5.1
 markdown-include==0.8.1
 MarkupSafe==2.1.3
 mkdocs==1.5.3