Bump django-mptt from 0.14.0 to 0.15.0

Bumps [django-mptt](https://github.com/django-mptt/django-mptt) from 0.14.0 to 0.15.0.
- [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@0.14...0.15)

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

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

example_project/requirements_test.txt

@@ -1,5 +1,5 @@
 beautifulsoup4==4.12.2
-django-mptt==0.14.0
+django-mptt==0.15.0
 django-webtest==1.9.11
 cssselect==1.2.0
 lxml==4.9.3