Support for django 4.0+

[TomaszKrassowski]

As per README: Currently, Django 2.2 (Python 3.7+) and Django 3.2 (Python 3.7+) are supported.

Will this work out of the box with Django 4.0? I do not see any features used that are removed in Django 4.0.

[nezhar]

AT the moment there are no known issues. The support needs to be updated in the action test workflow and README according to the current active support https://endoflife.date/django

@TomaszKrassowski if you want, you are welcome to open a PR with the changes :slightly_smiling_face:

[anx-mfischer]

https://github.com/beachmachine/django-userforeignkey/pull/17