Added wrapGAppsHook as nativeBuildInputs

I had

```
Traceback (most recent call last):
  File "/nix/store/sg2pb6pyl5w5jw9g2p145c0h6n7ngxvi-workspace-renumber", li
ne 2, in <module>
    from pye_menu import TopMenu, SubMenu, MenuItem, SubMenuItem, ExecMenuI
tem
  File "/nix/store/wq8m6gxcwbi5502javwkc7drvmh5sk7g-python3-3.7.5-env/lib/python3.7/site-packages/pye_menu.py", line 6, in <module>
    gi.require_versions({'Gtk': '3.0', 'PangoCairo': '1.0'})
  File "/nix/store/wq8m6gxcwbi5502javwkc7drvmh5sk7g-python3-3.7.5-env/lib/p
ython3.7/site-packages/gi/__init__.py", line 152, in require_versions
    require_version(module_name, module_version)
  File "/nix/store/wq8m6gxcwbi5502javwkc7drvmh5sk7g-python3-3.7.5-env/lib/p
ython3.7/site-packages/gi/__init__.py", line 129, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace Gtk not available
```

when trying to use it as a library. See

NixOS/nixpkgs#16285
NixOS/nixpkgs#56943

default.nix

@@ -3,13 +3,15 @@ in
 { python3 ? nixpkgs.python3,
   gtk3-x11 ? nixpkgs.gtk3-x11,
   gobjectIntrospection ? nixpkgs.gobjectIntrospection,
+  wrapGAppsHook ? nixpkgs.wrapGAppsHook,
   lib ? nixpkgs.lib 
 }:
 python3.pkgs.buildPythonPackage rec {
   pname = "pye-menu";
   version = "1.0";
   src = ./.;
   propagatedBuildInputs = with python3.pkgs; [ pygobject3 pycairo ];
+  nativeBuildInputs = [ wrapGAppsHook ];
   buildInputs = [ gtk3-x11 gobjectIntrospection ];
 
   meta = with lib; {