fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-11356592
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-11356593

Author: snyk-bot

requirements.txt

@@ -26,7 +26,7 @@ django==4.1.1
 gunicorn==20.1.0
 idna==3.4; python_version >= '3.5'
 lxml==4.9.1
-markdown2==2.4.3; python_version >= '3.5' and python_version < '4'
+markdown2==2.5.4; python_version >= '3.5' and python_version < '4'
 oauthlib==3.2.1; python_version >= '3.6'
 packaging==21.3; python_version >= '3.6'
 pillow==9.2.0