boundfield: Support Django 3

[jayvdb]

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This PR closes #

• [ ] Tests
• [ ] Docs/comments
  • [ ] IN CASE YOU'VE CHANGED THE formidable.yml file: run tox -e swagger-statics to rebuild the swagger static files and commit the diff.
• [ ] Migration(s)
• [ ] CHANGELOG.rst Updated
• [ ] Delete your branch

[jayvdb]

This is needed for https://build.opensuse.org/package/show/home:jayvdb:py-submit/python-django-formidable .