Bump markdown

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 3.7 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@3.7...3.8.1)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: 3.8.1
  dependency-type: indirect
...

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

Author: dependabot[bot]