Bump django from 3.1.4 to 3.1.6

Bumps [django](https://github.com/django/django) from 3.1.4 to 3.1.6.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@3.1.4...3.1.6)

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

requirements.txt

@@ -2,7 +2,7 @@ coverage==4.4.2
 django-admin-sortable2==0.7.7
 django-filer==2.0.2
 django-polymorphic==3.0.0
-Django==3.1.4
+Django==3.1.6
 easy_thumbnails==2.7.1
 future==0.18.2
 Markdown==2.6.11