fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGOSELECT2-10255155

Author: snyk-bot

requirements.txt

@@ -46,7 +46,7 @@ django-mptt==0.8.6
 #django-pgcrypto==1.4.0
 django-polymorphic>=2.0
 django-sekizai==0.10.0
-Django-Select2==4.3.2
+Django-Select2==8.4.1
 django-simple-menu==1.2.1
 django-taggit==0.18.3
 django-treebeard==4.1.0