Add riscv64 (linux_riscv64) wheel to PyPI releases

## Add `linux_riscv64` wheel to PyPI releases ### Summary `pip install pyzmq` on riscv64 Linux currently requires building from source (~5 min on a 1.6 GHz RISC-V SoC). Adding `linux_riscv64` to the wheel build matrix would give riscv64 users a prebuilt wheel. ### Evidence | | | |---|---| | **Tested wheel** | [`pyzmq-27.1.0-cp313-cp313-linux_riscv64.whl`](https://github.com/gounthar/riscv64-python-wheels/releases/download/v2026.03.11-cp313/pyzmq-27.1.0-cp313-cp313-linux_riscv64.whl) | | **Hardware** | BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz, 16 GB RAM) | | **Python** | 3.13 (CPython) | | **Build system** | cibuildwheel | | **Build time** | ~5 min (native, on hardware) | The wheel imports and passes basic smoke tests on riscv64 Linux. ### Suggested CI change **Workflow**: `.github/workflows/wheels.yml` Add riscv64 manylinux entry to the matrix: ```yaml # In matrix.include, add: - name: manylinux riscv64 os: ubuntu-24.04 cibw: arch: riscv64 build: '*manylinux*' ``` Add QEMU step: ```yaml - name: Set up QEMU if: contains(matrix.cibw.arch, 'riscv64') uses: docker/setup-qemu-action@v3 with: platforms: riscv64 ``` ### Ecosystem context - `manylinux_2_28_riscv64` images are available on `quay.io/pypa/` (landed in pypa/manylinux, 2025) - cibuildwheel 3.x supports riscv64 via QEMU emulation - auditwheel supports riscv64 platform tags - maturin-action supports `riscv64gc-unknown-linux-gnu` cross-compilation - Packages already shipping riscv64 wheels on PyPI: aiohttp, yarl, regex, markupsafe, charset-normalizer, rpds-py, multidict, propcache, watchfiles, rignore, setproctitle - RISC-V hardware is shipping: SiFive HiFive, SpacemiT K1/K3, Sophgo SG2044 (64-core) ### Our temporary index While upstream support is pending, we maintain a [PEP 503 index](https://gounthar.github.io/riscv64-python-wheels/simple/) with 50+ riscv64 wheels for the Python ML/AI stack, built natively on RISC-V hardware. Tracking repo: https://github.com/gounthar/riscv64-python-wheels