Bump markdown from 3.4.1 to 3.8.1

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 3.4.1 to 3.8.1.
- [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@3.4.1...3.8.1)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: 3.8.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
 pelican==4.8.0
-Markdown==3.4.1
+Markdown==3.8.1
 pelican-render-math==1.0.3
 beautifulsoup4==4.11.1