Add CI support and wheels for aarch64?

At present the only Linux target architecture we support is x86_64, but with the Raspberry Pi becoming more used it may be worth adding CI support for at least aarch64 (ARM v8 64-bit). This is not altogether straightforward, as this is not natively supported by github, as discussed in https://github.com/actions/runner-images/issues/5631, but it seems that QEMU _can_ be used to work around this, see https://github.com/uraimo/run-on-arch-action.