set difference strictly: fix #1840

Author: proost

stdlib/2and3/builtins.pyi

@@ -1039,8 +1039,8 @@ class set(MutableSet[_T], Generic[_T]):
     def __iand__(self, s: AbstractSet[object]) -> Set[_T]: ...
     def __or__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ior__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
-    def __sub__(self, s: AbstractSet[object]) -> Set[_T]: ...
-    def __isub__(self, s: AbstractSet[object]) -> Set[_T]: ...
+    def __sub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
+    def __isub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
     def __xor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ixor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __le__(self, s: AbstractSet[object]) -> bool: ...