swiften: add Python 3.12 for 2to3

2to3 was removed in Python 3.13.

https://github.com/NixOS/nixpkgs/issues/413845

pkgs/by-name/sw/swiften/package.nix

···
   zlib,
   fetchurl,
   fetchpatch,
+  python312,
   openssl,
   # pin Boost 1.86 due to use of boost/asio/io_service.hpp
   boost186,
···
   ];
 
   nativeBuildInputs = [
+    python312 # 2to3
     scons
   ];