add ReferenceType.__call__ to stubs

Author: BobTheBuidler

test-data/unit/lib-stub/weakref.pyi

@@ -9,6 +9,7 @@ _T = TypeVar("_T")
 class ReferenceType(Generic[_T]):  # "weakref"
     __callback__: Callable[[Self], Any]
     def __new__(cls, o: _T, callback: Callable[[Self], Any] | None = ..., /) -> Self: ...
+    def __call__(self) -> _T | None: ...
 
 ref = ReferenceType