Update dependency Django to v3.1.3

Author: renovate-bot

backend/requirements/base.txt

@@ -11,7 +11,7 @@ daphne==2.5.0
 graphene-django==2.13.0
 django-graphql-jwt==0.3.1
 
-Django==3.1.2
+Django==3.1.3
 django-filter==2.2.0
 djangorestframework==3.11.0
 djangorestframework_simplejwt==4.4.0