system: Fix app unregister function

Replace undefined 'instanceof' with 'isinstance'

Signed-off-by: Christian Romberg <distjubo@gmail.com>

wasp/wasp.py

@@ -176,7 +176,7 @@ class Manager():
 
     def unregister(self, cls):
         for app in self.launcher_ring:
-            if instanceof(app, cls):
+            if isinstance(app, cls):
                 self.launcher_ring.remove(app)
                 break