Bump django from 3.2.9 to 3.2.15 in /docs/_ext/djangodummy

Bumps [django](https://github.com/django/django) from 3.2.9 to 3.2.15.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@3.2.9...3.2.15)

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

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

Author: dependabot[bot]

docs/_ext/djangodummy/requirements.txt

@@ -1,6 +1,6 @@
 # for readthedocs
 # Remaining requirements are picked up from setup.py
-Django==3.2.9
+Django==3.2.15
 django-fluent-utils == 2.0.1
 django-mptt == 0.13.4
 django-parler == 2.2.1