fix failing stubtest

Author: jswhit2

src/netCDF4/__init__.pyi

@@ -388,7 +388,7 @@ class Dataset:
     def __getitem__(self, elem: str, /) -> Any: ...  # should be Group | Variable, but this causes too many problems
     # __iter__ and __contains__ always error because iteration and membership ops are not allowed
     def __iter__(self) -> NoReturn: ...
-    def __contains__(self, key) -> NoReturn: ...
+    def __contains__(self, key, /) -> NoReturn: ...
     def __setattr__(self, name: str, value: Any, /) -> None: ...
     def __getattr__(self, name: str, /) -> Any: ...
     def __delattr__(self, name: str, /): ...