_weakref.ReferenceType.__new__: all args are pos-only (#9820)

Fixes #9712

Author: AlexWaygood

stdlib/_weakref.pyi

@@ -20,7 +20,7 @@ class ProxyType(Generic[_T]):  # "weakproxy"
 
 class ReferenceType(Generic[_T]):
     __callback__: Callable[[ReferenceType[_T]], Any]
-    def __new__(cls, o: _T, callback: Callable[[ReferenceType[_T]], Any] | None = ...) -> Self: ...
+    def __new__(cls, __o: _T, __callback: Callable[[ReferenceType[_T]], Any] | None = ...) -> Self: ...
     def __call__(self) -> _T | None: ...
     if sys.version_info >= (3, 9):
         def __class_getitem__(cls, item: Any) -> GenericAlias: ...