Fix attribute loading for cases where the base class is not available

[alexanderlinne]

When the base class of an attribute as itself a subclass of Attribute, the base class may not be available. In this case, the ArchLoader creates an instance of Class instead of Attribute.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.63%. Comparing base (cfbfe15) to head (e96327f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
+ Coverage   77.61%   77.63%   +0.02%     
==========================================
  Files         258      258              
  Lines       19223    19226       +3     
  Branches     1606     1606              
==========================================
+ Hits        14919    14926       +7     
+ Misses       3847     3845       -2     
+ Partials      457      455       -2     

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