twtxt: use callPackage instead of python3Packages.callPackage

pkgs/top-level/all-packages.nix

···
     python3Packages.callPackage ../applications/misc/twitch-chat-downloader
       { };
 
-  twtxt = python3Packages.callPackage ../applications/networking/twtxt { };
+  twtxt = callPackage ../applications/networking/twtxt { };
 
   twurl = callPackage ../tools/misc/twurl { };