Wanting to use Gtk3 with Python3 on NixOS

I want to use Gtk3 in Python3 on NixOS. This command

nix-shell -p gobject-introspection gtk3 'python3.withPackages(ps : with ps; [ pygobject3 ])'

allows import gi in the python3 environment within the nix-shell. This is how I am trying to define Gtk for Python3 in the configuration.nix file.

  environment.systemPackages = with pkgs; [
    gtk3
    python3
    python3Packages.pygobject3
    python3Packages.pip
  ];

The sudo nixos-rebuild switch command does not show any error, but import gi within the python3 environment results in:

ModuleNotFoundError: No module named 'gi'

What do I need to change?

This is a fresh install of NixOs with the Cinnamon DE, Python 3.11.8, Gtk 3.24.

This is how I modified my configuration.nix file, thanks to the comment of @CharlesDuffy.

  users.users.reb = {
    isNormalUser = true;
    description = "RB";
    extraGroups = [ "networkmanager" "wheel" ];
    packages = with pkgs; [
      firefox
      libreoffice-fresh
      vlc
      gtk3
      (python3.withPackages (subpkgs: with subpkgs; [
        pip
        pygobject3
      ]))
    #  thunderbird
    ];
  };