klipper: configurable extra Python packages

With a view to making a plugin system in the NixOS Klipper module, this
lets us configure Klipper with extra Python packages, required for
plugins like klippain-shaketune.

pkgs/servers/klipper/default.nix

@@ -3,6 +3,7 @@
   lib,
   fetchFromGitHub,
   python3,
+  extraPythonPackages ? ps: [ ],
   unstableGitUpdater,
   makeWrapper,
   writeShellScript,
@@ -29,7 +30,9 @@ stdenv.mkDerivation rec {
 
   buildInputs = [
     (python3.withPackages (
-      p: with p; [
+      p:
+      with p;
+      [
         python-can
         cffi
         pyserial
@@ -38,6 +41,7 @@ stdenv.mkDerivation rec {
         markupsafe
         numpy
       ]
+      ++ extraPythonPackages p
     ))
   ];