pythonPackages.pyqt5: Add optional qtwebsockets support

This is e.g. required for gns3-gui 2.1.0. But it seems to break the
build for Python 2.7.

pkgs/development/python-modules/pyqt/5.x.nix

···
-{ lib, fetchurl, pythonPackages, pkgconfig, qtbase, qtsvg, qtwebkit, qtwebengine, dbus_libs
-, lndir, makeWrapper, qmake }:
+{ lib, fetchurl, pythonPackages, pkgconfig, makeWrapper, qmake
+, lndir, qtbase, qtsvg, qtwebkit, qtwebengine, dbus_libs
+, withWebSockets ? false, qtwebsockets
+}:
 
 let
   version = "5.9";
···
 
   buildInputs = [
     lndir qtbase qtsvg qtwebkit qtwebengine dbus_libs
-  ];
+  ] ++ lib.optional withWebSockets qtwebsockets;
 
   propagatedBuildInputs = [ sip ];