schismtracker: use python3

pkgs/applications/audio/schismtracker/default.nix

···
 , fetchFromGitHub
 , autoreconfHook
 , alsa-lib
-, python
+, python3
 , SDL
 }:
 
···
   configureFlags = [ "--enable-dependency-tracking" ]
     ++ lib.optional stdenv.isDarwin "--disable-sdltest";
 
-  nativeBuildInputs = [ autoreconfHook python ];
+  nativeBuildInputs = [ autoreconfHook python3 ];
 
   buildInputs = [ SDL ] ++ lib.optional stdenv.isLinux alsa-lib;