[PyUP] Update django-modelcluster to 4.2

[pyup-bot]

This PR updates django-modelcluster from 3.1 to 4.2.

Changelog

4.2

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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)

4.1

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

4.0

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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

Links

• PyPI: https://pypi.org/project/django-modelcluster
• Changelog: https://pyup.io/changelogs/django-modelcluster/
• Repo: https://github.com/wagtail/django-modelcluster

[pyup-bot]

Closing this in favor of #156