App: modernize C++: return braced init list

src/App/ExtensionPyImp.cpp

@@ -34,7 +34,7 @@ using namespace App;
 // returns a string which represent the object e.g. when printed in python
 std::string ExtensionPy::representation() const
 {
-    return std::string("<extension>");
+    return {"<extension>"};
 }
 
 PyObject *ExtensionPy::getCustomAttributes(const char* /*attr*/) const