Tweak keyword-only definition of AttributeIORef

This gives a nice reference of how to add KW_ONLY to the fields of child
classes so that they can have non-default fields, including positional
fields. Functionally it is the same.

Author: GDYendell

src/fastcs/attribute_io_ref.py

@@ -1,9 +1,9 @@
-from dataclasses import dataclass
+from dataclasses import KW_ONLY, dataclass
 
 from typing_extensions import TypeVar
 
 
-@dataclass(kw_only=True)
+@dataclass
 class AttributeIORef:
     """Base for references to define IO for an ``Attribute`` over an API.
 
@@ -14,6 +14,8 @@ class AttributeIORef:
     server.
     """
 
+    # Make fields keyword-only so that child classes can have fields without defaults
+    _: KW_ONLY
     update_period: float | None = None