xorg.xcbproto: do not depend on python2

pkgs/applications/misc/polybar/default.nix

@@ -24,8 +24,7 @@ assert nlSupport     -> ! iwSupport && libnl         != null;
 assert i3Support     -> ! i3GapsSupport && jsoncpp != null && i3      != null;
 assert i3GapsSupport -> ! i3Support     && jsoncpp != null && i3-gaps != null;
 
-let xcbproto-py3 = xcbproto.override { python = python3; };
-in stdenv.mkDerivation rec {
+stdenv.mkDerivation rec {
     pname = "polybar";
     version = "3.4.1";
 
@@ -51,7 +50,7 @@ in stdenv.mkDerivation rec {
     };
 
     buildInputs = [
-      cairo libXdmcp libpthreadstubs libxcb pcre python3 xcbproto-py3 xcbutil
+      cairo libXdmcp libpthreadstubs libxcb pcre python3 xcbproto xcbutil
       xcbutilcursor xcbutilimage xcbutilrenderutil xcbutilwm xcbutilxrm
 
       (if alsaSupport   then alsaLib       else null)

pkgs/servers/x11/xorg/overrides.nix

@@ -4,7 +4,7 @@
   freetype, tradcpp, fontconfig, meson, ninja, ed,
   libGL, spice-protocol, zlib, libGLU, dbus, libunwind, libdrm,
   mesa, udev, bootstrap_cmds, bison, flex, clangStdenv, autoreconfHook,
-  mcpp, epoxy, openssl, pkgconfig, llvm_6,
+  mcpp, epoxy, openssl, pkgconfig, llvm_6, python3,
   ApplicationServices, Carbon, Cocoa, Xplugin
 }:
 
@@ -308,6 +308,10 @@ self: super:
     buildInputs = attrs.buildInputs ++ [ freetype fontconfig ];
   });
 
+  xcbproto = super.xcbproto.override {
+    python = python3;
+  };
+
   xcbutil = super.xcbutil.overrideAttrs (attrs: {
     outputs = [ "out" "dev" ];
   });