Update django-modelcluster to 6.0

6.0 (14.03.2022)
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* BREAKING: ClusterForm now builds no child formsets when neither `formsets` nor `exclude_formsets` is specified in the Meta class, rather than building a formset for every child relation (Matt Westcott)
* Removed Python 3.5 and 3.6 support
* Removed Django 2.0 and 2.1 support
* Support explicit definitions for nested formsets within ClusterForm, via a `formsets` option on the outer formset's definition (Matt Westcott)
* Add `inherit_kwargs` attribute to ClusterForm child formsets (Matt Westcott)

5.3 (10.03.2022)
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* Avoid accessing live queryset on unsaved instances, for preliminary Django 4.1 compatibility (Matt Westcott)
* Support traversing one-to-one and many-to-one relations in `filter` / `order_by` lookups (Andy Babic)
* Implement `values()` method on FakeQuerySet (Andy Babic)
* Allow `values()` and `values_list()` to be chained with other queryset modifiers (Andy Babic)
* Fix: Fix HTML escaping behaviour on `ClusterForm.as_p()` (Matt Westcott)
* Fix: Match standard behaviour queryset of returning foreign keys as IDs in `values_list` (Andy Babic)

5.2 (13.10.2021)
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* Implement `copy_cluster` method on ClusterableModel (Karl Hobley)
* Add `formset_name` option on ClusterableModel formsets to allow the formset name to differ from the relation name (Matt Westcott)
* Fix: Fix tests for Django 3.2 (Alex Tomkins, Matt Westcott, Mar鱈a Fernanda Magallanes)
* Fix: Ensure ptr_id fields are correctly populated when deserialising models with multi-level inheritance (Alex Tomkins)

(joerg)