Looks great, thanks for sharing! I'd definitely try this approach.

Do you write default.nix files by hand, or there are some tools to automate the process?

I suppose for a project with a lot of dependencies (one of my current requirements.txt is over 100 SLOC long - almost all deps are on PyPI; haven't checked nixpkgs as I'm stuck with Debian at the moment) it would be somewhat tiresome to check every package and for every missing one list all the dependencies.