Use OCCT Handle macro for Handle_ classes

This is the result of running OCCT's upgrade script provided
with OCCT 7.0. See
https://www.opencascade.com/content/freecad-occt710-and-windows-rtti-data-missing#comment-form
and
https://www.forum.freecadweb.org/viewtopic.php?f=4&t=21405&start=120#p169019
for why this is necessary for OCCT >= 7.1

src/Mod/MeshPart/App/Mesher.cpp

@@ -172,7 +172,7 @@ Mesh::MeshObject* Mesher::createMesh() const
 {
     // OCC standard mesher
     if (method == Standard) {
-        Handle_StlMesh_Mesh aMesh = new StlMesh_Mesh();
+        Handle(StlMesh_Mesh) aMesh = new StlMesh_Mesh();
 
         if (!shape.IsNull()) {
             BRepTools::Clean(shape);