update shell.nix

shell.nix

@@ -2,7 +2,7 @@ let
   pkgs = import <nixpkgs> {};
 in
   pkgs.mkShell {
-    buildInputs = [
+    buildInputs = with pkgs;[
 
       #pkgs.texlive.combined.scheme-small 
 
@@ -39,6 +39,9 @@ in
         # if available, just add it to the above list 
 
       })
-    ];
+      python3
+    ] ++ (with python3Packages; [
+      pygments 
+    ]);
   }