Django-marina

- Switch to a CalVer YY.MINOR versioning scheme. MINOR is the number of the release in the given year. This is the first release in 2021 using this scheme, so its version is 21.1. The next version this year will be 21.2. The first version in 2022 will be 22.1.
- Fix status code error message (110).
- Add support for Django 4.0 and Python 3.10 (82).
- Remove `pur` from developer requirements.

- Allow `assertAllowed`, `assertForbidden`, `assertNotFound` to test for multiple users in a single call.
- Only allow user None to trigger logout.

- Make `assertLoginRequired` test for redirect to `LOGIN_URL`.
- Improve Makefile build and publish commands.

- Add `ExtendedTestCase.assertHasMessage` to test messages in response.
- Fix bug in ProtectedModelMixin.
- Add tests.

- Add Dependabot.
- Revert to setuptools for packaging.
- Drop support for Django 3.0, extended support stopped on 2021-04-01).
- Add support for Django 3.2.

- Fix CHANGELOG.