Update django-filter to 21.1

[pyup-bot]

This PR updates django-filter from 2.4.0 to 21.1.

Changelog

21.1

------------------------

This is a maintenance release updating CI testing for the latest
non-end-of-life versions of Python and Django, and updating package metadata
accordingly.

With this release ``django-filter`` is switching to a two-part CalVer
versioning scheme, such as ``21.1``. The first number is the year. The second
is the release number within that year.

On an on-going basis, Django-Filter aims to support all current Django
versions, the matching current Python versions, and the latest version of
Django REST Framework.

Please see:

* `Status of supported Python branches <https://devguide.python.org/#status-of-python-branches>`_
* `List of supported Django versions <https://www.djangoproject.com/download/#support-versions>`_

Support for Python and Django versions will be dropped when they reach
end-of-life. Support for Python versions will dropped when they reach
end-of-life, even when still supported by a current version of Django.

Other breaking changes are rare. Where required, every effort will be made to
apply a "Year plus two" deprecation period. For example, a change initially
introduced in ``23.x`` would offer a fallback where feasible and finally be
removed in ``25.1``. Where fallbacks are not feasible, breaking changes without
deprecation will be called out in the release notes.

Beyond that change, there are few changes. Some small bugfixes, improvements to
localisation, and documentation tweaks. Thanks to all who were involved.

Links

• PyPI: https://pypi.org/project/django-filter
• Changelog: https://pyup.io/changelogs/django-filter/
• Repo: https://github.com/carltongibson/django-filter/tree/main

[pyup-bot]

Closing this in favor of #134