Update avl_tree.py

Author: lighting9999

data_structures/binary_tree/avl_tree.py

@@ -8,8 +8,10 @@
 
 from __future__ import annotations
 
+import doctest
 import math
 import random
+
 from typing import Any
 
 
@@ -330,7 +332,6 @@ def __str__(
 
 
 def _test() -> None:
-    import doctest
 
     doctest.testmod()