Replace load staticfiles with load static

[ArianK16a]

This was deprecated in django 2.1 and removed in django 3.0 https://docs.djangoproject.com/en/4.2/releases/2.1/#id2 https://docs.djangoproject.com/en/4.2/releases/3.0/#features-removed-in-3-0

[ArianK16a]

Hey @danielw92 - do you think there's any chance to get this merged and a new tag out?
This should be fine to use since Django 1.10 (https://docs.djangoproject.com/en/4.2/releases/1.10/#django-contrib-staticfiles) and it's necessary for this package to work on Django 3.0+.
So far it seems to work good on Django 4.2 with just this change.