Switch poetry nix shell to use python39

templates/poetry.nix

@@ -1,16 +1,9 @@
-{ pkgs ? import <nixpkgs> {}, python ? pkgs.python3, ... }:
+{ pkgs ? import <nixpkgs> {}, ... }:
 
-let
-  python-packages = ps: with ps; [
-    pip
-    setuptools
-    wheel
-  ];
-in
 pkgs.mkShell {
-  nativeBuildInputs = with pkgs; [
-    (python.withPackages python-packages)
-    poetry
+  packages = with pkgs; [
+    python39
+    (poetry.override { python3 = python39; })
     git
     zlib
     libgccjit
@@ -21,7 +14,7 @@ pkgs.mkShell {
   shellHook = ''
     export LD_LIBRARY_PATH="${pkgs.glibc}/lib:${pkgs.glibc_multi}/lib:${pkgs.zlib}/lib:${pkgs.libgccjit}/lib"
     [ ! -f "pyproject.toml" ] && poetry init -n
-    poetry env use "$(command -v ${python}/bin/python)"
+    poetry env use "$(command -v ${pkgs.python39}/bin/python)"
     [ -d "./.venv" ] && source .venv/bin/activate
   '';
 }