check also for PBuiltinMethod if the underlying function takes a self arg explicitly

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/call/special/CallSpecialMethodNode.java

@@ -121,6 +121,8 @@ protected static boolean takesSelfArg(Object func) {
             if (functionRootNode instanceof BuiltinFunctionRootNode) {
                 return ((BuiltinFunctionRootNode) functionRootNode).declaresExplicitSelf();
             }
+        } else if (func instanceof PBuiltinMethod) {
+            return takesSelfArg(((PBuiltinMethod) func).getFunction());
         }
         return true;
     }