setup.py: fix import error reporting for cythonize import, see #8208 (master)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.76%. Comparing base (7baf8be) to head (ce2a824). Report is 1 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8215 +/- ## ======================================= Coverage 83.75% 83.76% ======================================= Files 67 67 Lines 12061 12061 Branches 2189 2189 ======================================= + Hits 10102 10103 +1 Misses 1367 1367 + Partials 592 591 -1 ```

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