[Security] Bump markdown2 from 2.3.6 to 2.3.9

Bumps [markdown2](https://github.com/trentm/python-markdown2) from 2.3.6 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@2.3.6...2.3.9)

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

requirements.txt

@@ -34,7 +34,7 @@ django-related-admin==0.5.0
 django-ajax-selects==1.9.0
 django-kronos==1.0
 httpsig==1.2.0
-markdown2==2.3.6
+markdown2==2.3.9
 timeout-decorator==0.4.0
 phonenumbers==8.9.0
 isort==4.2.15