[Security] Bump markdown2 from 2.3.5 to 2.3.9

Bumps [markdown2](https://github.com/trentm/python-markdown2) from 2.3.5 to 2.3.9. **This update includes security fixes.**
- [Release notes](https://github.com/trentm/python-markdown2/releases)
- [Changelog](https://github.com/trentm/python-markdown2/blob/master/CHANGES.md)
- [Commits](trentm/python-markdown2@2.3.5...2.3.9)

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

requirements/base.txt

@@ -14,7 +14,7 @@ git+https://github.com/ministryofjustice/cla_common.git@0.3.8#egg=cla_common
 speaklater==1.3 # Required by cla_common
 itsdangerous==0.24
 logstash_formatter==0.5.9
-markdown2==2.3.5
+markdown2==2.3.9
 python-dateutil==2.2
 pytz==2018.5
 requests==2.19.1