text: Title: 'DataTree' object does not support the context manager protocol
  Body: ### What is your issue?
  
  ```python
  ds = xr.Dataset(
      {"foo": (("x", "y"), np.random.rand(4, 5))},
      coords={
          "x": [10, 20, 30, 40],
          "y": pd.date_range("2000-01-01", periods=5),
          "z": ("x", list("abcd")),
      },
  )
  ds.to_netcdf("saved_on_disk.nc")
  
  # works as intended
  with xr.open_dataset("saved_on_disk.nc") as ds:
      print(ds)
  
  # doesn't work
  with xr.open_datatree("saved_on_disk.nc") as dtree:
      print(dtree)
  ```
  
  ```
  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  Cell In[5], line 1
  ----> 1 with xr.open_datatree("saved_on_disk.nc") as dtree:
        2     print(dtree)
  
  TypeError: 'DataTree' object does not support the context manager protocol
  ```
  