[17:54:09] jon@nixos ~/projects/nixpkgs (python39)
$ GC_INITIAL_HEAP_SIZE=16g nix-env -f ./outpaths.nix -qaP --no-name --out-path --arg checkMeta true --show-trace > out-paths
[18:01:15] jon@nixos ~/projects/nixpkgs (python39)

let
  unstableTarball = fetchTarball
    "https://github.com/NixOS/nixpkgs/archive/204f52ce65513c0448a283959902fb166adc39f5.tar.gz";
  pkgs = import <nixpkgs> { };
  unstable = import unstableTarball {
    overlays = [
      (self: super: {
        pythonOverrides = python-self: python-super: {

          # Boto doesn't work with python3.9
          boto = python-super.boto3;
          moto = python-super.moto.overrideAttrs( x: {
            # Don't make moto require boto, since boto doesn't work with python3.9
            prePatch = ''
                substituteInPlace setup.py --replace "boto>=2.36.0" ""
            '';
          });
          # pull pyflakes from master
          pyflakes = python-super.pyflakes.overrideAttrs (x: {
            src = super.fetchFromGitHub {
              owner = "PyCQA";
              repo = "pyflakes";
              rev = "26cf0631fe89f61d5b0ef8d6949676f051e35796";
              sha256 = "sha256-F/dQpM7j/MOHOpteUten2ttK78lvwMMmNVOpAFo2gPk=";
            };
          });
        };
        python39 =
          super.python39.override { packageOverrides = self.pythonOverrides; };
      })
    ];
  };
...