Bump django from 3.1.12 to 3.1.13 in /demo

Bumps [django](https://github.com/django/django) from 3.1.12 to 3.1.13.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@3.1.12...3.1.13)

---
updated-dependencies:
- dependency-name: django
  dependency-type: direct:production
...

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

demo/requirements.txt

@@ -1,2 +1,2 @@
-Django==3.1.12
+Django==3.1.13
 django-sitetree==1.14