Python: Modernise ShouldBeContextManager

Author: RasmusWL

python/ql/src/Classes/ShouldBeContextManager.ql

@@ -14,8 +14,8 @@
 
 import python
 
-from ClassObject c
-where not c.isC() and not c.isContextManager() and exists(c.declaredAttribute("__del__"))
+from ClassValue c
+where not c.isBuiltin() and not c.isContextManager() and exists(c.declaredAttribute("__del__"))
 select c,
     "Class " + c.getName() +
         " implements __del__ (presumably to release some resource). Consider making it a context manager."