DM-52492: Use dict.fromkeys rather than dict comprehension

[timj]

Checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.38%. Comparing base (2ff75da) to head (7008d81).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   32.38%   32.38%           
=======================================
  Files          12       12           
  Lines        1130     1130           
  Branches      189      189           
=======================================
  Hits          366      366           
  Misses        752      752           
  Partials       12       12           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.