[deps] Update markdown requirement from ~=3.8.2 to ~=3.9

Updates the requirements on [markdown](https://github.com/Python-Markdown/markdown) to permit the latest version.
- [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@3.8.2...3.9.0)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: '3.9'
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
 openwisp-users @ https://github.com/openwisp/openwisp-users/tarball/1.2
 openwisp-utils[rest,celery,channels] @ https://github.com/openwisp/openwisp-utils/tarball/1.2
-markdown~=3.8.2
+markdown~=3.9
 jsonfield>=2.1.0