Update AttributeIO to call Tracer __init__

This makes it clearer that child classes need to call super init

Author: GDYendell

src/fastcs/attribute_io.py

@@ -26,6 +26,9 @@ def __init_subclass__(cls) -> None:
         args = get_args(cast(Any, cls).__orig_bases__[0])
         cls.ref_type = args[1]
 
+    def __init__(self):
+        super().__init__()
+
     async def update(self, attr: AttrR[T, AttributeIORefT]) -> None:
         raise NotImplementedError()