Update django-model-utils to 3.1.2

[pyup-bot]

This PR updates django-model-utils from 2.5.2 to 3.1.2.

Changelog

3.1.2

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* Update InheritanceIterable to inherit from
ModelIterable instead of BaseIterable, fixes GH-277.

3.1.1

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- Update classifiers and README via GH-306, fixes GH-305

3.1.0

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- Support for Django 2.0 via GH-298, fixes GH-297
- Remove old travis script via GH-300
- Fix codecov and switch to py.test 301

3.0.0

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* Drop support for Python 2.6.
* Drop support for Django 1.4, 1.5, 1.6, 1.7.
* Exclude tests from the distribution, fixes GH-258.
* Add support for Django 1.11 GH-269

2.6.1

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* Fix infinite recursion with multiple `MonitorField` and `defer()` or `only()`
on Django 1.10+. Thanks Romain Garrigues. Merge of GH-242, fixes GH-241.

* Fix `InheritanceManager` and `SoftDeletableManager` to respect
`self._queryset_class` instead of hardcoding the queryset class. Merge of
GH-250, fixes GH-249.

* Add mixins for `SoftDeletableQuerySet` and `SoftDeletableManager`, as stated
in the the documentation.

* Fix `SoftDeletableModel.delete()` to use the correct database connection.
Merge of GH-239.

* Added boolean keyword argument `soft` to `SoftDeletableModel.delete()` that
revert to default behavior when set to `False`. Merge of GH-240.

* Enforced default manager in `StatusModel` to avoid manager order issues when
using abstract models that redefine `objects` manager. Merge of GH-253, fixes
GH-251.

2.6

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* Added `SoftDeletableModel` abstract class, its manageer
`SoftDeletableManager` and queryset `SoftDeletableQuerySet`.

* Fix issue with field tracker and deferred FileField for Django 1.10.

Links

• PyPI: https://pypi.org/project/django-model-utils
• Changelog: https://pyup.io/changelogs/django-model-utils/
• Repo: https://github.com/jazzband/django-model-utils/