Update django-model-utils to 4.0.0

[pyup-bot]

This PR updates django-model-utils from 3.1.2 to 4.0.0.

Changelog

### 4.0.0 ``` ------------------ - Added `Choices.subset`. - Remove hacks for previously supported Django versions. (Fixes GH-390) - Dropped support for Python 2.7. (Fixes GH-393) - Dropped usage of `six` - Drop support for `Django 1.11` - Add support for `Python 3.8` - Add support for `Django 3.0` ``` ### 3.2.0 ``` ------------------- - Catch `AttributeError` for deferred abstract fields, fixes GH-331. - Update documentation to explain usage of `timeframed` model manager, fixes GH-118 - Honor `OneToOneField.parent_link=False`. - Fix handling of deferred attributes on Django 1.10+, fixes GH-278 - Fix `FieldTracker.has_changed()` and `FieldTracker.previous()` to return correct responses for deferred fields. - Add Simplified Chinese translations. - Update AutoLastModifiedField so that at instance creation it will always be set equal to created to make querying easier. Fixes GH-254 - Support `reversed` for all kinds of `Choices` objects, fixes GH-309 - Fix Model instance non picklable GH-330 - Fix patched `save` in FieldTracker - Upgrades test requirements (pytest, pytest-django, pytest-cov) and skips tox test with Python 3.5 and Django (trunk) - Add UUIDModel and UUIDField support. ```

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/

[coveralls]

Pull Request Test Coverage Report for Build 1251

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.275%

---

Totals
Change from base Build 1198:	0.0%
Covered Lines:	2191
Relevant Lines:	2207

---

💛 - Coveralls

[pyup-bot]

Closing this in favor of #389