App: modernize C++: return braced init list

src/App/GroupExtensionPyImp.cpp

@@ -36,7 +36,7 @@ using namespace App;
 // returns a string which represent the object e.g. when printed in python
 std::string GroupExtensionPy::representation() const
 {
-    return std::string("<group extension object>");
+    return {"<group extension object>"};
 }
 
 PyObject*  GroupExtensionPy::newObject(PyObject *args)