remove imports from deprecated scipy.ndimage.filters namespace

[moustakas]

Unit tests have myriad deprecation warnings of the sort:

/home/runner/work/desispec/desispec/py/desispec/preproc.py:14: DeprecationWarning: 
Please use `median_filter` from the `scipy.ndimage` namespace, the `scipy.ndimage.filters` 
namespace is deprecated.
    from scipy.ndimage.filters import median_filter

https://github.com/desihub/desispec/actions/runs/3919211294/jobs/6700069463#step:5:65

The scipy.ndimage.filters namespace was deprecated as far back as scipy/0.19.1 (I think), so I didn't bother to support both new and old versions.

[weaverba137]

Looks fine to me.