Update python venv template to be more robust

Move template into python39 folder in case I add more in the future

templates/python39/shell.nix

@@ -0,0 +1,8 @@
+{pkgs ? import <nixpkgs> {}, ...}:
+pkgs.mkShell {
+  nativeBuildInputs = with pkgs; [python39 poetry git];
+  shellHook = ''
+    [ -e .venv/bin/python3 ] && VENV_FLAG="--upgrade" || VENV_FLAG="--clear"; python3 -m venv $VENV_FLAG .venv
+    [ -e .venv/bin/activate ] && source .venv/bin/activate
+  '';
+}

templates/venv.nix

@@ -1,12 +0,0 @@
-{
-  pkgs ? import <nixpkgs> {},
-  python ? pkgs.python39,
-  ...
-}:
-pkgs.mkShell {
-  nativeBuildInputs = with pkgs; [(python.withPackages (ps: with ps; [pip wheel])) poetry git];
-  shellHook = ''
-    [ -d .venv ] || python -m venv .venv
-    source .venv/bin/activate
-  '';
-}