Bump django-polymorphic from 2.0.3 to 2.1.2

Bumps [django-polymorphic](https://github.com/django-polymorphic/django-polymorphic) from 2.0.3 to 2.1.2.
- [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@v2.0.3...v2.1.2)

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

requirements.txt

@@ -31,7 +31,7 @@ django-modeltranslation>=0.11,<0.13.3
 django-treebeard==4.3
 django-guardian<=1.4.9
 django-downloadview<=1.9
-django-polymorphic==2.0.3
+django-polymorphic==2.1.2
 django-tastypie<=0.14.0
 oauthlib==3.0.2
 pyopenssl==19.0.0