Use PEP 604 syntax wherever possible (#7493)

stdlib/threading.pyi

@@ -1,9 +1,9 @@
 import sys
 from types import FrameType, TracebackType
-from typing import Any, Callable, Iterable, Mapping, Optional, TypeVar
+from typing import Any, Callable, Iterable, Mapping, TypeVar
 
 # TODO recursive type
-_TF = Callable[[FrameType, str, Any], Optional[Callable[..., Any]]]
+_TF = Callable[[FrameType, str, Any], Callable[..., Any] | None]
 
 _PF = Callable[[FrameType, str, Any], None]
 _T = TypeVar("_T")