Pscycopg2-binary is normally used in Python development to avoid having to build psycopg2 from source.
In nixpkgs we always want ot build from source whenever possible, but it can still be useful to provide a psycopg2-binary package.
This "fake" package exists to satisfy a dependency on psycopg2-binary, but still use the build from psycopg2.
cc @misuzu https://github.com/nix-community/pyproject.nix/issues/143