Django-marina

- Reformat CHANGELOG.
- Add Python 3.9 to test matrix.
- Update Django 3.1 to non-development version in test matrix.
- Switch CI to GitHub Actions.
- Add docs and tests to sdist.

- Add string argument to assertContainsSelector and assertNotContainsSelector in ExtendedTestCase.
- Add documentation on ExtendedTestCase.
- Package sphinx_rtd_theme is now properly installed as a `docs` extra.

- Add Django 3.1 to test matrix.
- Fix coveralls.

- Fix documentation on ReadTheDocs.
- Fix coveralls shield in README.
- Default branch of repository renamed `main`, related files have been updated.
- Update documentation and Sphinx configuration.
- Simplify imports and add tests.
- Update tox configuration.

- Use [poetry](https://python-poetry.org) for managing dependencies and packaging.
- Extra test functions with `django_marina.test.clients.ExtendedClient` and `django_marina.test.test_cases.ExtendedTestCase`.
- Generic way to protect model instances against deletion using `django_marina.db.models.ProtectedModelMixin`.
- Skip migrations (useful in tests) with `django_marina.db.migrations.NoMigrations`.

- Not meant for public release.