libxslt: use python3 on darwin

See #63174 where the condition was first introduced.

pkgs/top-level/all-packages.nix

···
   libxmp = callPackage ../development/libraries/libxmp { };
 
   libxslt = callPackage ../development/libraries/libxslt {
-    python = if stdenv.isDarwin then python2 else python3;
+    python = python3;
   };
 
   libxsmm = callPackage ../development/libraries/libxsmm { };