return typehint for test func updated

Author: sephml

Diff for: machine_learning/mab.py

@@ -342,7 +342,7 @@ def update(self, arm_index: int, reward: int) -> None:
         self.values[arm_index] += (reward - self.values[arm_index]) / n
 
 
-def test_mab_strategies():
+def test_mab_strategies() -> None:
     """
     Test the MAB strategies.
     """