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- add support for Django 4.x
- fix rendering of hidden fields in `bootstrap_form`
- overwrite the default form template so that `{{ form }}` will produce the
same output as `{% bootstrap_form form %}` (starting with Django 4.0)
- starting with Django 4.0, make use of `widget.use_fieldset`
- use same font size for `<legend>` as for `<label>`
- work around extensive spacing between legend and multiwidget