DM-43101: Fix type annotation of _serializedType property

[timj]

Without this the SupportsSimple protocol wouldn't match things like DatasetRef.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.92%. Comparing base (9f02822) to head (36c40a2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #986      +/-   ##
==========================================
- Coverage   88.92%   88.92%   -0.01%     
==========================================
  Files         329      329              
  Lines       42351    42351              
  Branches     8704     8704              
==========================================
- Hits        37661    37660       -1     
  Misses       3441     3441              
- Partials     1249     1250       +1     

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