Updated _ParameterBase.__init_subclass__ to call the superclass's `…

…__init_subclass__` method. (#969)

param/parameterized.py

@@ -971,6 +971,7 @@ def _modified_slots_defaults(cls):
 
     @classmethod
     def __init_subclass__(cls):
+        super().__init_subclass__()
         # _update_signature has been tested against the Parameters available
         # in Param, we don't want to break the Parameters created elsewhere
         # so wrapping this in a loose try/except.