failing upstream-dev polyfit warning test

[keewis]

In xarray/tests/test_dataset.py::TestDataset::test_polyfit_warnings, the second call to polyfit:

ds.var1.polyfit("dim2", 10, full=True)

emits a warning with the current upstream-dev pandas:

xarray/tests/test_dataset.py::TestDataset::test_polyfit_warnings
  .../xarray/core/alignment.py:307: FutureWarning: Index.__or__ operating as a set operation is deprecated, in the future this will be a logical operation matching Series.__or__.  Use index.union(other) instead
    index = joiner(matching_indexes)

since the test makes sure only the first call emits a warning (a np.RankWarning), the test fails.

[mathause]

Our alignment is based on this. So this may have quite some implications. I am surprised there are not more warnings (by number) in the test suite.

For reference: pandas-dev/pandas#37374