Fix resolution of extension classes that have references

[rly]

Motivation

Fix #1182

How to test the behavior?

pytest

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.01%. Comparing base (d378dec) to head (a9dd391).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1183      +/-   ##
==========================================
+ Coverage   88.92%   89.01%   +0.09%     
==========================================
  Files          45       45              
  Lines        9866     9872       +6     
  Branches     2806     2810       +4     
==========================================
+ Hits         8773     8788      +15     
+ Misses        776      767       -9     
  Partials      317      317              

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