spidev: add build-spidev.yml for riscv64 wheels

[luhenry]

• Package: spidev
• Version: 3.8
• Source: http://github.com/doceme/py-spidev
• Docs: http://github.com/doceme/py-spidev

Compiles a single C extension (spidev) wrapping the Linux /dev/spidev* ioctl interface for user-space SPI access. Upstream publishes no riscv64 wheel.

Upstream ships no CI workflow and no test suite; this workflow just checks the extension out at the release tag and builds it with cibuildwheel.

Differs from upstream

• Nothing beyond the riscv64 image.

Testing

• Upstream has none, and the riscv64 runner has no real /dev/spidev* device, so the wheel is exercised by importing it, checking __version__, and asserting open_path raises OSError for a missing device.

License: OK - MIT, wheel ships LICENSE only.