Django-organizations

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* Adds migrations to support Django 4.x
* Removes support for Django < 3.2
* Updates tox testing for supported configurations of Django 3.2 - 4.1 with Python 3.7 - 3.10
* Early tox testing of Django 4.1 with Python 3.11
* Fixes GitHub Actions automated testing

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* Remove `default_app_config` for forward compatiblity with Django 4.1

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* Better compatibility with Django 3.2

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* Invitation model backend uses models to track invitations
* Registration/inviation backends take an optional namespace argument on initialization. The use
case is if you want to namespace the URLs
* Can provide a dotted path to `invitation_backend` and `registration_backend` functions
* Drops support for Python 2 and Django versions < 2.2 LTS
* Migrate the codebase to an src/ layout
* Now with more test coverage!

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* Fixes issue with default backend where users defined without first/last names
might not be represented

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* Migrations and slug related fixup

This is a small but significant change. A change introduced in version 1.0.0 due to
incompatability with an unmaintained default dependency yielded migration issues for
many users. This release switches *back* to a *fork* of the original dependency
to revert the default changes.