[Security] Bump markdown2 from 2.3.7 to 2.3.9

Bumps [markdown2](https://github.com/trentm/python-markdown2) from 2.3.7 to 2.3.9. **This update includes a security fix.**
- [Release notes](https://github.com/trentm/python-markdown2/releases)
- [Changelog](https://github.com/trentm/python-markdown2/blob/master/CHANGES.md)
- [Commits](trentm/python-markdown2@2.3.7...2.3.9)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -28,7 +28,7 @@ idna==2.8
     # via requests
 lxml==3.7.1
     # via http-crawler
-markdown2==2.3.7
+markdown2==2.3.9
     # via django-markdown-deux
 pyyaml==5.4
     # via django-amber