Update django-model-utils to 4.1.1

[pyup-bot]

This PR updates django-model-utils from 4.0.0 to 4.1.1.

Changelog

4.1.1

------------------
- Applied `isort` to codebase (Refs GH-402)
- Fix `TypeError` in save when model inherits from both TimeStampModel
and StatusModel. (Fixes GH-465)

4.1.0

------------------
- Update InheritanceQuerySetMixin to avoid querying too much tables
- TimeStampedModel now automatically adds 'modified' field as an update_fields
parameter even if it is forgotten while using save()
- `FieldTracker` now marks fields as not changed after `refresh_from_db`
- `FieldTracker` now respects `update_fields` changed in overridden `save()`
method
- Replace ugettext_lazy with gettext_lazy to satisfy Django deprecation warning
- Add available_objects manager to SoftDeletableModel and add deprecation
warning to objects manager.
- StatusModel now automatically adds 'status_changed' field during save as an
update_fieldsparameter when 'status' is present in it to make sure it is not
forgotten.
- Update test requirements
- Move tests to GitHub Actions: https://github.com/jazzband/django-model-utils/actions
- Drop support for `Django 2.1`
- Add support for `Python 3.9`
- Add support for `Django 3.1`

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