Django-bootstrap4

- Convert HISTORY.rst to Markdown and rename to CHANGELOG.md.
- Convert README.rst, AUTHORS.rst and CONTRIBUTING.rst to Markdown, and change extension to .md.
- Update CONTRIBUTING.md to reflect use of `poetry`.
- Drop contributing.rst from documentation.
- Rename default branch 'master' to 'main'.

- Set beautifulsoup4 requirement to \>= 4.8.0 (fixes 216).
- Do not assume all inputs are inside labels (fixes 215).

- Use poetry (<https://python-poetry.org/>) for dependency management and packaging.
- Drop support for Python 3.5.
- Fix form-check-{label,input} classes applied too broadly. These should only be applied to labels and inputs found underneath the enclosing widget div for radio and checkbox select, not on the whole document.
- Allow to display radio and checkbox elements inline using the `form-check` classes.

- Restructure tox and Makefile.
- Add test for `bootstrap4.__version__`.

- Remove tag command from Makefile.
- Reinstate `bootstrap4.__version__` (186).
- Add test for using a different jQuery version (184).

- Update default Bootstrap to v4.3.1.
- Add support for Python 3.8, Django 3 and Django master.
- Switch to Django `manage.py` for test running.
- Update Makefile commands.
- Update tox configuration.
- Use correct license (BSD-3-Clause).
- Fix typo's in docstrings.
- Update Travis configuration.
- Drop MANIFEST.in, use `setuptools_scm`.
- Stop using `_version.py`, use git tags for versioning.
- Fixed issues with labels and input (174 and 181).