setprofile accepts None (#3239)

The function `threading.setprofile` can be called with a `None` value to
clear the profile function. This does unset the function from the
existing threads, it only clears the callback function so that new
threads are created without the profiling hook installed.

stdlib/2and3/threading.pyi

@@ -30,7 +30,7 @@ if sys.version_info >= (3, 4):
     def main_thread() -> Thread: ...
 
 def settrace(func: _TF) -> None: ...
-def setprofile(func: _PF) -> None: ...
+def setprofile(func: Optional[_PF]) -> None: ...
 def stack_size(size: int = ...) -> int: ...
 
 if sys.version_info >= (3,):