Python: Restrict ShouldBeContextManager.ql results

By limiting the results to the class that actually defines the `__del__`
method, we eliminate a bunch of FPs where a _subclass_ of such a class
would also get flagged.

python/ql/src/Classes/ShouldBeContextManager.ql

@@ -19,7 +19,7 @@ private import semmle.python.dataflow.new.internal.DataFlowDispatch
 from Class c
 where
   not DuckTyping::isContextManager(c) and
-  DuckTyping::hasMethod(c, "__del__")
+  exists(c.getMethod("__del__"))
 select c,
   "Class " + c.getName() +
     " implements __del__ (presumably to release some resource). Consider making it a context manager."