return type of `Dataset.dims` will be changed

[mathause]

A deprecation notice from xarray:

The return type of Dataset.dims will be changed to return a set of dimension names in future, in order to be more consistent with DataArray.dims. To access a mapping from dimension names to lengths, please use Dataset.sizes.

This is e.g. used here

ConTrack/contrack/contrack.py

Line 128 in 501293b

return self.ds.dims[self._get_name_time()]