Specify minimum versions in setup.cfg

[dcherian]

See #5342 (comment)

[mathause]

Is there a way to do this for optional dependecies?

[keewis]

it apparently is possible to add versions for optional dependencies, but unlike the required dependencies those will only be interpreted when doing something like pip install xarray[complete].

For conda I guess we would have to create multiple packages (following dask and matplotlib that would be xarray-core and xarray)?