Python: Fix all metrics-related compilation failures

In hindsight, having a `.getMetrics()` method that just returns `this`
is somewhat weird. It's possible that it predates the existence of the
inline cast, however.

python/ql/src/Functions/OverlyComplexDelMethod.ql

@@ -18,6 +18,6 @@ from FunctionValue method
 where
   exists(ClassValue c |
     c.declaredAttribute("__del__") = method and
-    method.getScope().getMetrics().getCyclomaticComplexity() > 3
+    method.getScope().(FunctionMetrics).getCyclomaticComplexity() > 3
   )
 select method, "Overly complex '__del__' method."