Support additional dtypes in resample

[oliverhiggs]

pandas.BaseOffset, pandas.Timedelta, datetime.timedelta, and BaseCFTimeOffset are now all supported datatypes for resampling.

Closes#9408

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[max-sixty]

Looks good! Can we add a test?

[oliverhiggs]

This is failing a test on the bare-minimum environment because of a FutureWarning caused by pandas 2.0.3 using 'S' instead of 's' for an internal frequency string. I noticed there are other FutureWarnings that occur during the test run but don't cause a test failure, why is this? Is it possible to do the same with my test?

e.g. This warning does not cause a test failure.

 /home/runner/work/xarray/xarray/xarray/tests/test_variable.py:2421: FutureWarning: the `pandas.MultiIndex` object(s) passed as 'x' coordinate(s) or data variable(s) will no longer be implicitly promoted and wrapped into multiple indexed coordinates in the future (i.e., one coordinate for each multi-index level + one dimension coordinate). If you want to keep this behavior, you need to first wrap it explicitly using `mindex_coords = xarray.Coordinates.from_pandas_multiindex(mindex_obj, 'dim')` and pass it as coordinates, e.g., `xarray.Dataset(coords=mindex_coords)`, `dataset.assign_coords(mindex_coords)` or `dataarray.assign_coords(mindex_coords)`.
ds = Dataset(coords={"x": midx})

[dcherian]

yes we can take it from here. Thanks for your contribution and welcome to Xarray!

[oliverhiggs]

Thanks! And thanks for creating such a welcoming atmosphere for new contributors.

[spencerkclark]

@oliverhiggs sorry for the delay—this looks great!

To answer your question about the warning leading to a test failure—a few months ago we made a change to make any new warnings emitted in the test suite errors (#8974). In that PR some code was added to ignore some existing warnings (the MultiIndex warning you noted being one of them).

We've dealt with the frequency string deprecation warning in the past in the context of old pandas versions (#8627); I made a suggestion which I think may resolve it.

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