App: modernize C++: return braced init list

src/App/PartPyImp.cpp

@@ -32,7 +32,7 @@ using namespace App;
 // returns a string which represents the object e.g. when printed in python
 std::string PartPy::representation() const
 {
-    return std::string("<Part object>");
+    return {"<Part object>"};
 }
 
 PyObject *PartPy::getCustomAttributes(const char* /*attr*/) const