Django-modelcluster

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* Fix: ClusterForm without an explicit `formsets` kwarg now allows formsets to be omitted from form submissions, to fix regression with nested relations

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* Django 2.2 compatibility
* Support nested child relationships in ClusterForm (Sam Costigan)

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* Added support for filter lookup expressions such as `__lt`

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* Django 2.1 compatibility
* Python 3.7 compatibility
* Implemented prefetch_related on FakeQuerySet (Haydn Greatnews)
* Fix: Saving a ClusterableModel with a primary key of 0 no longer throws an IntegrityError (A Lee)
* Fix: Serialization now respects `serialize=False` on ParentalManyToManyFields (Tadas Dailyda)

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* `on_delete` on ParentalKey now defaults to CASCADE if not specified

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* Django 2.0 compatibility
* Removed Django 1.8 and 1.9 support
* Child formsets now validate uniqueness constraints
* Fix: Many-to-many relations inside inline formsets are now saved correctly