fix: include python3 in Nix flake, needed for Tilt (#122)

default.nix

@@ -26,6 +26,7 @@ in pkgs.mkShell {
     lsof
     pkg-config
     openssl.dev
+    python3
     (writeShellScriptBin "fish" ''
       exec ${pkgs.fish}/bin/fish -C 'mise activate fish | source' "$@"
     '')
@@ -53,4 +54,4 @@ in pkgs.mkShell {
       echo "✅ Playwright versions in nix and npm are the same"
     fi
   '';
-}
+}