Django-bootstrap4

- Switch to a [CalVer](https://calver.org) 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.
- Add support for Django 4.0 and Python 3.10 (349).
- Fix faulty example code (347).

- No line break in FileInput in horizontal layout.
- Actually use the bundles JavaScript.

- Drop support for Django 3.0, extended support stopped on 2021-04-01).
- Add support for Django 3.2.
- Use bundled Bootstrap JavaScript, no need for separate popper.js.
- Updated default Bootstrap to 4.6.0.
- Add Dependabot for updates to dependencies.
- Rename AUTHORS.md to AUTHORS, remove authors section from documentation.
- Revert to setuptools for packaging.
- Add Python 3.9 to Travis CI.
- Add docs and tests to sdist.
- Use GitHub Actions for CI.
- Fix example by not installing editable version.

- Fix CHANGELOG.

- Updated default Bootstrap to 4.5.2.
- Updated CSS/JavaScript URLs to newer versions (thanks emmceemoore).
- Replace `m2r` with `m2r2` to support Sphinx3.
- Update Sphinx dependency because of security update.
- Use Django 3.1 in `tox` matrix, fix warning in tests.
- Add `tox` to development dependencies.
- Accept importlib-metadata 2.x.x (thanks dbaty).
- Add Python 3.9 to tox matrix.

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