ARM runners don't have PyPy

[henryiii]

Our action default includes pypy-3.10, which is missing for builds like ubuntu-arm-24.04. It's also probably not present on the other images, which means it has to be downloaded, making the action slower. I think we could just remove pypy from the default, like other projects are doing (cibuildwheel 3, for example). Someone who needs it could ask for it (as they already need to do for older pypy). Then our default would also work on the ARM images. I'd assume they might eventually build it for ARM, but this seems pretty reasonable overall?