Update markdown requirement from <3,>=2.6.11 to >=2.6.11,<4

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)
- [Commits](Python-Markdown/markdown@2.6.11...3.2.2)

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

test_reqs.txt

@@ -1,5 +1,5 @@
 django-treebeard==4.3.1
-Markdown>=2.6.11,<3
+Markdown>=2.6.11,<4
 coverage==5.1
 flake8==3.8.2
 django-jenkins==0.110.0