[deps] Update markdown requirement from ~=3.4.3 to ~=3.6

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@3.4.3...3.6)

---
updated-dependencies:
- dependency-name: markdown
  dependency-type: direct:production
...

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

requirements.txt

@@ -2,4 +2,4 @@ django-notifications-hq~=1.8.2
 channels~=3.0.2
 openwisp-users @ https://github.com/openwisp/openwisp-users/tarball/master
 openwisp-utils[rest,celery] @ https://github.com/openwisp/openwisp-utils/tarball/master
-markdown~=3.4.3
+markdown~=3.6