generalize is_fileset_or_union utility to is_subclass_or_union

[tclose]

Types of changes

• New feature (non-breaking change which adds functionality)

Summary

Generalises a small typing union is_fileset_or_union to is_subclass_or_union. is_fileset_or_union is still there but simply calls is_subclass_or_union

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.28%. Comparing base (fc3a6ce) to head (39478a0).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #842      +/-   ##
==========================================
- Coverage   88.49%   88.28%   -0.21%     
==========================================
  Files          86       86              
  Lines       17988    17990       +2     
  Branches     3509     3509              
==========================================
- Hits        15919    15883      -36     
- Misses       1689     1729      +40     
+ Partials      380      378       -2     

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