python.pkgs.pyqtwebengine: init at 5.12.1

5.13.0 is available, however, it assumes qt 5.13 is used and fails
to build due to a missing QWebEngineNotification class.

pkgs/top-level/python-packages.nix

@@ -949,6 +949,10 @@ in {
   */
   pyqt5_with_qtwebkit = self.pyqt5.override { withWebKit = true; };
 
+  pyqtwebengine = pkgs.libsForQt5.callPackage ../development/python-modules/pyqtwebengine {
+    pythonPackages = self;
+  };
+
   pysc2 = callPackage ../development/python-modules/pysc2 { };
 
   pyscard = callPackage ../development/python-modules/pyscard { inherit (pkgs.darwin.apple_sdk.frameworks) PCSC; };