Add support for pyodide wheels

cibuildwheel now supports ``pyodide_wasm32`` wheels - it would be good to make sure we support this properly - I managed to get it work by doing:

```
      targets: |
        - cp*-pyodide_wasm32
      env: |
        CIBW_PLATFORM: pyodide
```

but would be good if the platform could get set automatically.