www/py-dj42-django-modelcluster: Add py-dj42-django-modelcluster 6.2.1 (copied...

www/py-dj42-django-modelcluster: Add py-dj42-django-modelcluster 6.2.1 (copied from py-django-modelcluster)

django-modelcluster extends Django's foreign key relations to make this
possible. It introduces a new type of relation, ParentalKey, where the related
models are stored locally to the 'parent' model until the parent is explicitly
saved. Up to that point, the related models can still be accessed through a
subset of the QuerySet API.