[threading] trace functions can be None (#15182)

Author: nedbat

stdlib/threading.pyi

@@ -54,12 +54,12 @@ def currentThread() -> Thread: ...
 def get_ident() -> int: ...
 def enumerate() -> list[Thread]: ...
 def main_thread() -> Thread: ...
-def settrace(func: TraceFunction) -> None: ...
+def settrace(func: TraceFunction | None) -> None: ...
 def setprofile(func: ProfileFunction | None) -> None: ...
 
 if sys.version_info >= (3, 12):
     def setprofile_all_threads(func: ProfileFunction | None) -> None: ...
-    def settrace_all_threads(func: TraceFunction) -> None: ...
+    def settrace_all_threads(func: TraceFunction | None) -> None: ...
 
 if sys.version_info >= (3, 10):
     def gettrace() -> TraceFunction | None: ...