fix fast string creation for pypy

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #75 will not alter performance

_{Comparing pypy-fix (e8dda13) with main (496e36a)}

Summary

✅ 59 untouched benchmarks

[codecov[bot]]

Codecov Report

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

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