There are some different/new tools for creating your own Python packages these days. It's still not truly solved in the sense of having a single clear winner , but one of these new package generation tools might serve you better:
poetry2nix: https://github.com/nix-community/poetry2nix
mach-nix: https://github.com/DavHau/mach-nix
dream2nix: https://nix-community.github.io/dream2nix/guides/getting-sta...
pynixify: https://github.com/cript0nauta/pynixify
pip2nix: https://github.com/nix-community/pip2nix
The tools available to you at the time (pypi2nix and maybe python2nix, if it was a long time ago) have been abandoned in favor of the newer tools, I think chiefly poetry2nix but I'm not sure.
There's still the Nixpkgs buildPythonPackage stuff, I think, if your goal is to upstream a lib into Nixpkgs. But if you just want to build your own Python applications and vendorize the deps (e.g., for work), you might try one of the tools above, which weren't available 3+ years ago.
dream2nix is by the author of mach-nix IIRC and has the goal of establishing a unified standard and codebase for ${proglang}2nix type package generators. But mach-nix is still maintained and might be the more feature-complete choice between them.
Maybe Nix-y Python users and developers can reply with some of their experiences using those tools for real projects :)