Django-bootstrap4

- 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).

- Fix support for Python 3.5 (168).
- Set correct Python versions in setup.py.
- Fix `make test` command.

- Set default class for buttons to `btn-primary` (150.
- Drop support for Python 2 and Django 1.11, 2.0 and 2.1 (155).
- Template tag `bootstrap_field` now allows 3 values for `show_label`: `True`, `False`, `sr-only` and `skip\`. In the case of `False` / `sr-only` the label is hidden but present for screen readers. When `show_label` is set to `skip`, the label is not generated at all.
- Fix validation on input groups (122).
- No longer duplicate jquery in bootstrap4.html (139, 140).
- Apply `form-check`, `form-check-label` and `form-check-input` classes to appropriate tags for `RadioSelect` and `CheckboxSelectMultiple` widgets (141).
- Errors on file inputs are shown (146).
- Only display non-field errors in form alert block (161).
- Reinstate `bootstrap4_title` block to `bootstrap4.html` (156).
- Fix typo in `alert-dismissible` class (142).
- Honor the `form_group_class` parameter in the `buttons` tag (162).

- Drop support for the `base_url` setting (105).
- Remove use of \"falsy\" as a string literal (111).
- Fix javascript inclusion bugs (71).
- Allow email to have addons (131).
- Do not mark placeholder text as safe (135).

- Improve alert accessibility (93.
- Add a new widget: RadioSelectButtonGroup (103.
- Several fixes and improvements to HTML renderin.
- Switch to explicit Travis test.
- Upgrade Bootstrap to 4.1.1
- Upgrade jQuery to 3.3.1
- Upgrade Popper to 1.14.3
- Fixed `bootstrap4.html` to add jQuery per setting `include_jquery=[False|'full'|'slim']`.
- Adopt Black formatting, see <https://github.com/ambv/black>.

- Change form help text element from div to small (60).
- Upgrade Bootstrap to 4.0.0 (66).