Django-admin-sort

- Fix regression introduced in 1.0.1, adding double item rows on
SortableInlineAdminMixin and TabularInline.

- Fix CSS classes change introduced in Django-2.1.
- Prepared to run on Django-4.0.
- Ditch Travis-CI in favor of GitHub Actions.

- Drop support for Python-2.7, 3.4 and 3.5.
- Drop support for Django-1.10, 1.11, 2.0 and 2.1.
- Add Python-3.9 to the testing matrix.
- Refactor code base to clean Python-3 syntax.

- Fix [\207](https://github.com/jrief/django-admin-sortable2/issues/207):
Last item not displayed in stacked- and tabular inline admins, if model doesn\'t have add permission.

- Add support for Django-3.1.

- Fix [\241](https://github.com/jrief/django-admin-sortable2/issues/241):
Move items when the order column is not first.
- Fix [\242](https://github.com/jrief/django-admin-sortable2/issues/242):
Bulk move when sorting order is descending.
- Fix [\243](https://github.com/jrief/django-admin-sortable2/issues/243):
Bulk move to last page when it is too small.
- Refactor aggregates to use Coalasce for empty querysets.