fix _pydantic_core.pyi to be compatible with 3.7

Author: samuelcolvin

python/pydantic_core/_pydantic_core.pyi

@@ -54,7 +54,7 @@ class Some(Generic[_T]):
     @property
     def value(self) -> _T: ...
     @classmethod
-    def __class_getitem__(cls, item: Any, /) -> Type[Self]: ...
+    def __class_getitem__(cls, __item: Any) -> Type[Self]: ...
 
 @final
 class SchemaValidator: