Bump django-polymorphic from 3.0.0 to 3.1.0

Bumps [django-polymorphic](https://github.com/django-polymorphic/django-polymorphic) from 3.0.0 to 3.1.0.
- [Release notes](https://github.com/django-polymorphic/django-polymorphic/releases)
- [Changelog](https://github.com/jazzband/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
  update-type: version-update:semver-minor
...

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

base-requirements.txt

@@ -47,7 +47,7 @@ django-ordered-model==3.4.3
 django-widget-tweaks==1.4.8
 django-countries==7.2.1
 num2words==0.5.10
-django-polymorphic==3.0.0
+django-polymorphic==3.1.0
 sorl-thumbnail==12.7.0
 django-extensions==3.1.4
 django-import-export==2.7.1