Django-bootstrap3

- Revert of 453, which turned out to break checkboxes (fixes 467)
- Update requirements and fix `make docs`
- Replace `force_text` with `force_str`, removes warnings

- Reinstate ``bootstrap3.__version__`` (fixes 486)
- Update Makefile, travis and tox configuration (470)

- Drop support for Python 2.7, Django 1.11 and Django 2.0 (456)
- Fix Deprecationwarning in Python 3.7 (455)
- Add label class support to form field checkboxes (453)
- Move development tasks from `setup.py` to `Makefile`
- Fix compatibility with Django 3.0 and master
- Add Django 3.0 to `tox.ini`
- Update versions in `requirements.txt`
- Use Makefile for common tasks
- Drop `MANIFEST.in`, use `setuptools_scm`
- Drop `_version.py`, use version from git tag

- Update Bootstrap to 3.4.1 (459)
- **NOTE** Version 12 will drop support for Python 2.x.x and Django 1.x.x

- Support `crossorigin` and `integrity` in urls (443)
- Switch to explicit Travis tests (444)
- Fix PyPI classifiers
- Remove obsolete code for Django <= 1.8 (446)
- Remove obsolete settings `set_required` and `set_disabled` (445)
- Remove setting `base_url` (443)

- Fix PyPI classifiers