Update kd_node.py

Author: cclauss

data_structures/kd_tree/kd_node.py

@@ -11,8 +11,8 @@ class KDNode:
     def __init__(
         self,
         point: list[float],
-        left: KDNode | None = None,
-        right: KDNode | None = None,
+        left: "KDNode" | None = None,
+        right: "KDNode" | None = None,
     ) -> None:
         """
         Initializes a KDNode with the given point and child nodes.