Django-polymorphic-tree

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* Added Django 4.0 compatibility.
* Fixed permission check for moving nodes.
* Dropped Python 3.5 support.

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* Fixed Django 3.1 compatibility.
* Fixed Django 4.x deprecation warnings.
* Bumped third party requirements to meaningful versions.
* Dropped Django 1.10, 1.11, 2.0 support.
* Dropped Python 2.7 support.

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* Fixed Django 3.0 compatibility by removing ``django.utils.six`` dependency.

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* Added Django 2.0 support.
* Added ``base_manager_name`` setting in model ``Meta`` options, like django-polymorphic_ 2.0 does.
* Fixed crash in ``get_child_types()`` when ``can_have_children`` is ``False``.
* Fixed child type validation, this now happens before the database saves.
* Fixed spelling grapelli -> grappelli
* Dropped Django 1.8 support.

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* Fixed compatibility with recent django-polymorphic_ releases.
* Dropped support for unmaintained Django versions: 1.6, 1.7, 1.9
(this should have warranted a 1.5 release, but it was accidentally slipped in).

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* Fixed inheriting ``PolymorphicMPTTModel`` in an abstract model.
* Fixed ``PolymorphicMPTTQuerySet.as_manager()`` usage to return the proper manager.