chore(deps): update django-polymorphic requirement

Updates the requirements on [django-polymorphic](https://github.com/django-polymorphic/django-polymorphic) to permit the latest version.
- [Release notes](https://github.com/django-polymorphic/django-polymorphic/releases)
- [Changelog](https://github.com/django-polymorphic/django-polymorphic/blob/master/docs/changelog.rst)
- [Commits](jazzband/django-polymorphic@3.0.0...v3.1)

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

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

Author: dependabot[bot]

requirements.txt

@@ -7,7 +7,7 @@ celery==5.0.5
 Django>=2.0,<3.0
 django-extensions
 django-hurricane==0.8.8
-django-polymorphic~=3.0.0
+django-polymorphic~=3.1.0
 djangorestframework~=3.12.1
 environs~=8.0.0
 gitpython~=3.1.9