DM-20970 (hotfix): Add astropy dependency to setup.cfg

[timj]

Otherwise pip install won't realize it is needed.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #114 (04bc355) into main (79a716f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #114   +/-   ##
=======================================
  Coverage   91.86%   91.86%           
=======================================
  Files          40       40           
  Lines        2532     2532           
=======================================
  Hits         2326     2326           
  Misses        206      206           

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