Add support for Django 4.0

[Melevir]

It would be great if django-waffle is Django 4.0 compatible.

Sadly, new version of Django dropped a lot of deprecated functions, so update required some actions in django-waffle's code.

Here are two problems:

• NullBooleanField usage in migration (check out #441)
• ImportError: cannot import name 'url' from 'django.conf.urls'

This one because django.conf.urls.url was removed in Django 4.0 and it's imported in waffle/urls.py.

[rh0dium]

Also

• [ ] Remove default_app_config

If you need help holler!

[clintonb]

@rh0dium a pull request is welcome!

[Melevir]

I see that the main branch works fine with django4. Any plans on the PyPI release?

[clintonb]

@Melevir feel free to open a pull request that adds test support.

[Melevir]

That was my plan, but it turns out, that the only problem, that blocked me from running waffle on django4 was URLs thing, which is resolved in https://github.com/django-waffle/django-waffle/commit/c781c32de8d7e823e54486862ce5f70d2f49da33

What else blocks django-waffle from django4-compatible release? I'll be glad to help.

[clintonb]

• [ ] Add explicit test support for 4.0 at https://github.com/django-waffle/django-waffle/blob/master/tox.ini#L19
• [ ] Update classifiers in https://github.com/django-waffle/django-waffle/blob/master/setup.py#L27

[clintonb]

Fixed by #421