Update to django 4.0
Things I don't love
- Using unreleased version of djstripe, should switch to a released version asap https://github.com/dj-stripe/dj-stripe/issues/1507
- Using my own fork on django-bitfield https://github.com/disqus/django-bitfield/pull/123
Generally merits more testing but could be merged. If we merge with those forked versions, let's open issues to remind ourselves to change it back later.