Mypy failures with MultiIndex

[jsignell]

What is your issue?

As seen in https://github.com/pydata/xarray/actions/runs/13771308824/job/38510332904?pr=10109 and https://github.com/pydata/xarray/actions/runs/13771399760/job/38510636156?pr=10062

 xarray/core/indexes.py: note: In member "stack" of class "PandasMultiIndex":
xarray/core/indexes.py:1051: error: Argument 2 to "MultiIndex" has incompatible type "list[ndarray[Any, dtype[Any]]]"; expected "Sequence[Sequence[int]]"  [arg-type]
Generated Cobertura report: /home/runner/work/xarray/xarray/mypy_report/cobertura.xml
xarray/core/indexes.py: note: In member "from_variables_maybe_expand" of class "PandasMultiIndex":
xarray/core/indexes.py:1123: error: Argument 2 to "MultiIndex" has incompatible type "list[Iterable[int]]"; expected "Sequence[Sequence[int]]"  [arg-type]
xarray/core/coordinates.py: note: In member "to_index" of class "AbstractCoordinates":
xarray/core/coordinates.py:186: error: Argument 2 to "MultiIndex" has incompatible type "list[ndarray[Any, dtype[Any]]]"; expected "Sequence[Sequence[int]]"  [arg-type]
xarray/core/groupby.py: note: In member "factorize" of class "ComposedGrouper":
xarray/core/groupby.py:530: error: Argument 1 to "from_product" of "MultiIndex" has incompatible type "Generator[ndarray[Any, Any], None, None]"; expected "Sequence[Any]"  [arg-type]
xarray/tests/test_dataset.py: note: In member "test_drop_indexes" of class "TestDataset":
xarray/tests/test_dataset.py:2926: error: List item 0 has incompatible type "list[int]"; expected "tuple[Hashable, ...]"  [list-item]
xarray/tests/test_dataset.py:2926: error: List item 1 has incompatible type "list[int]"; expected "tuple[Hashable, ...]"  [list-item]
xarray/tests/test_dataset.py: note: In member "test_rename_multiindex" of class "TestDataset":
xarray/tests/test_dataset.py:3222: error: List item 0 has incompatible type "list[int]"; expected "tuple[Hashable, ...]"  [list-item]
xarray/tests/test_dataset.py:3222: error: List item 1 has incompatible type "list[int]"; expected "tuple[Hashable, ...]"  [list-item]
Installing missing stub packages:
/home/runner/micromamba/envs/xarray-tests/bin/python -m pip install types-openpyxl


Generated Cobertura report: /home/runner/work/xarray/xarray/mypy_report/cobertura.xml
Found 8 errors in 4 files (checked 177 source files)