[Security] Bump markdown2 from 2.3.8 to 2.3.9

Bumps [markdown2](https://github.com/trentm/python-markdown2) from 2.3.8 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@2.3.8...2.3.9)

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