qtwebkit: use python38 to build instead of python2

python38 appears to work just as well, so it seems better to use that rather
than python2.  This also resolves some build flakiness seen due to parallel
invocations of python2 that are fixed in python3.

Note that the scripts aren't compatible with python39 or later, some patching
would be required to resolve that.

pkgs/development/libraries/qt-5/modules/qtwebkit.nix

@@ -2,7 +2,7 @@
 , qtbase, qtdeclarative, qtlocation, qtmultimedia, qtsensors, qtwebchannel
 , fontconfig, libwebp, libxml2, libxslt
 , sqlite, systemd, glib, gst_all_1, cmake
-, bison, flex, gdb, gperf, perl, pkg-config, python2, ruby
+, bison, flex, gdb, gperf, perl, pkg-config, python38, ruby
 , ICU, OpenGL
 }:
 
@@ -30,7 +30,7 @@ qtModule {
     ++ lib.optionals stdenv.isDarwin [ ICU OpenGL ]
     ++ lib.optional usingAnnulenWebkitFork hyphen;
   nativeBuildInputs = [
-    bison flex gdb gperf perl pkg-config python2 ruby
+    bison flex gdb gperf perl pkg-config python38 ruby
   ] ++ lib.optional usingAnnulenWebkitFork cmake;
 
   cmakeFlags = lib.optionals usingAnnulenWebkitFork ([ "-DPORT=Qt" ]