Set django polymorphic version to newest (#1290)

Author: sliverc

requirements/requirements-optionals.txt

@@ -1,5 +1,2 @@
 django-filter==24.3
-# once next version has been released (>3.1.0) this
-# should be set to pinned version again
-# see https://github.com/django-polymorphic/django-polymorphic/pull/541
-django-polymorphic@git+https://github.com/django-polymorphic/django-polymorphic@master  # pyup: ignore
+django-polymorphic==4.1.0