Remove scikit-image runtime dependency

[karlhillx]

ccdproc's block_reduce/block_average/block_replicate wrappers all delegate to astropy.nddata. scikit-image was never imported at runtime -- only in the test file, and only for a version-check skip condition that used broken string comparison.

Removed it from runtime deps and cleaned up the test imports.

[mwcraig]

Thanks for the quick fix @karlhillx -- can you add yourself to the list of contributors or would you like me to do that for you?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.56%. Comparing base (2659867) to head (16dcaa1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #923   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files           8        8           
  Lines        1571     1571           
=======================================
  Hits         1517     1517           
  Misses         54       54           

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[karlhillx]

Thanks for the quick fix @karlhillx -- can you add yourself to the list of contributors or would you like me to do that for you?

Done, just pushed a commit adding myself. Thanks Matt!

[mwcraig]

Thanks @karlhillx!