paperless wasn't building because django-cors-headers didn't
build anymore with the pinned django version.
I tested the paperless service (editd a few documents) and it works
fine, even with django unpinned.
`django-cors-headers` 3.x (which is used in nixpkgs) requires a scheme
for allowed hosts. Upstream uses 2.4, however we create the python env
with Nix, so the source needs to be patched accordingly.
