Drop py39

[seddonym]

Drop support for Python 3.9, as it is now end-of-life.

[codspeed-hq]

CodSpeed Performance Report

Merging #261 will not alter performance

_{Comparing drop-py39 (06a831d) with main (045c175)}

Summary

✅ 26 untouched
⏩ 23 skipped¹

Footnotes

1. 23 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩