fix doctests: pandas 2.1 MultiIndex repr with nan (#8141)

Co-authored-by: Mathias Hauser <mathause@users.noreply.github.com>

Author: benbovy

xarray/core/dataarray.py

@@ -2904,9 +2904,9 @@ def to_unstacked_dataset(self, dim: Hashable, level: int | Hashable = 0) -> Data
             b        (x) int64 0 3
         >>> stacked = data.to_stacked_array("z", ["x"])
         >>> stacked.indexes["z"]
-        MultiIndex([('a', 0.0),
-                    ('a', 1.0),
-                    ('a', 2.0),
+        MultiIndex([('a',   0),
+                    ('a',   1),
+                    ('a',   2),
                     ('b', nan)],
                    name='z')
         >>> roundtripped = stacked.to_unstacked_dataset(dim="z")