Upgrade pairwise implementation in tests to itertools

[nathanjmcdougall]

[ ] Add tests for the change. In general, aim for full test coverage at the Python level. Rust tests are optional.
[ ] Add any appropriate documentation.
[ ] Add a summary of changes to the latest section at the top of CHANGELOG.rst. (If it's not there, add it.)
[ ] Add your name to AUTHORS.rst.
[ ] Run just full-check.

[codspeed-hq]

CodSpeed Performance Report

Merging #287 will not alter performance

_{Comparing nathanjmcdougall:maint/pairwise-upgrade (dc977ec) with main (ce83089)}

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. ↩