fix type annotations for 'optuna/_callbacks.py'

Author: boringbyte

optuna/_callbacks.py

@@ -1,5 +1,6 @@
+from __future__ import annotations
+
 from typing import Container
-from typing import Optional
 
 import optuna
 from optuna.trial import FrozenTrial
@@ -45,7 +46,7 @@ def objective(trial):
     """
 
     def __init__(
-        self, n_trials: int, states: Optional[Container[TrialState]] = (TrialState.COMPLETE,)
+        self, n_trials: int, states: Container[TrialState] | None = (TrialState.COMPLETE,)
     ) -> None:
         self._n_trials = n_trials
         self._states = states