Fix sys.setprofile annotation. (#1679)

`sys.setprofile(None)` is valid, and is in fact the only way to clear a profiler once set.

stdlib/3/sys.pyi

@@ -143,7 +143,7 @@ def _getframe(depth: int) -> FrameType: ...
 
 _ProfileFunc = Callable[[FrameType, str, Any], Any]
 def getprofile() -> Optional[_ProfileFunc]: ...
-def setprofile(profilefunc: _ProfileFunc) -> None: ...
+def setprofile(profilefunc: Optional[_ProfileFunc]) -> None: ...
 
 _TraceFunc = Callable[[FrameType, str, Any], Optional[Callable[[FrameType, str, Any], Any]]]
 def gettrace() -> Optional[_TraceFunc]: ...