Verify update fails if run before pin

[pinch] / shell.nix

{ pkgs ? import <nixpkgs> { } }:
pkgs.mkShell rec {
  doCheck = true;
  buildInputs = with pkgs; [ (python3.withPackages (ps: with ps; [ xdg ])) ];
  checkInputs = with pkgs; [
    mypy
    python3Packages.autopep8
    python3Packages.pylint
  ];
}