Bump django-mptt from 0.13.3 to 0.13.4

Bumps [django-mptt](https://github.com/django-mptt/django-mptt) from 0.13.3 to 0.13.4.
- [Release notes](https://github.com/django-mptt/django-mptt/releases)
- [Changelog](https://github.com/django-mptt/django-mptt/blob/main/CHANGELOG.rst)
- [Commits](django-mptt/django-mptt@0.13.3...0.13.4)

---
updated-dependencies:
- dependency-name: django-mptt
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

tests/requirements.txt

@@ -5,7 +5,7 @@ Django==2.2.24
 django-ajax-selects==1.9.1
 django-ical==1.8.0
 django-paypal==1.0.0
-django-mptt==0.13.3
+django-mptt==0.13.4
 django-post-office==3.5.3
 django-timezone-field==4.2.1
 djangorestframework==3.12.4