fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGORESTFRAMEWORK-7252137

requirements.txt

@@ -50,7 +50,7 @@ Django-Select2==4.3.2
 django-simple-menu==1.2.1
 django-taggit==0.18.3
 django-treebeard==4.1.0
-djangorestframework>=3.9.1
+djangorestframework>=3.15.2
 docopt==0.6.2
 docutils==0.12
 #dodgy==0.1.9