webapp: Fix backport list
To actually be able to install python3-swh.web.
Prior to this commit:
python3-swh.web : Depends: python3-django (>= 1:1.11.0~) but it is not going to be installed
Depends: python3-django-js-reverse but it is not going to be installed
Depends: python3-django-webpack-loader but it is not going to be installed
Depends: python3-djangorestframework (>= 3.4.0~) but it is not going to be installed