Django-bootstrap4

- 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.

- Fix date in CHANGELOG.
- Fix typo in pyproject.toml (222).