Tests fail under PyPy

[ericsnowcurrently]

See: https://github.com/python/pyperformance/runs/5725083064

I'm not sure how long it's been a problem.

[hugovk]

Tests have been passing on PyPy for some time. For example:

https://github.com/python/pyperformance/actions/runs/7807808563/job/21296909152