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/

[pyup-bot]

Closing this in favor of #482