text: Title: open_datatree `group` parameter does not support Iterable[str] 
  Body: ### What is your issue?
  
  The annotation typing for the  `group` parameter in the `open_datatree` function receives `str | Iterable[str] | Callable | None = None` for all backends. for example
  
  https://github.com/pydata/xarray/blob/f01096fef402485092c7132dfd042cc8f467ed09/xarray/backends/zarr.py#L1285
  
  However, the current implementation does not support `Iterable[str]`; Therefore, it should be removed in all backends
  
  
  
  