Bump django from 3.1.6 to 3.1.8 in /src/backend

Bumps [django](https://github.com/django/django) from 3.1.6 to 3.1.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.6...3.1.8)

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

src/backend/requirements.txt

@@ -2,7 +2,7 @@ requests-oauth==0.4.1
 requests-oauthlib==1.3.0
 requests==2.23.0
 
-django==3.1.6
+django==3.1.8
 django-storages==1.9.1
 django-filter==2.2.0
 django-cors-headers==3.2.1