System - NixOS

Hello, i've trying to pyvips to work in a python venv.

Installation skips to cffi ABI mode, fails there too. But i want to get the API mode.

What does it exactly mean to have libvips with dev headers installed?
From the trace i can see that "import _libvips" couldn't any. Where does it look?

Nix have vips package, but no -dev version. I could make my own -dev version of it, but how does it differ.

Thanks!