ci: add riscv64 to wheel build matrix

[gounthar]

Summary

Add riscv64 to the cibuildwheel arch matrix so that linux_riscv64 wheels are built and published to PyPI.

Changes:

• Add riscv64 to cibw_arch list
• Exclude riscv64 on macOS and Windows (Linux-only)
• Extend QEMU setup condition to include riscv64
• Add riscv64 to QEMU platforms

The existing QEMU infrastructure (already used for aarch64) handles emulation.

Evidence

• Tested wheel: httptools-0.7.1-cp313-cp313-linux_riscv64.whl
• Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz)
• Build time: ~2 min
• Imports and passes basic smoke test on riscv64

Context

• manylinux_2_28_riscv64 images available (pypa/manylinux, 2025)
• cibuildwheel 3.x supports riscv64 via QEMU
• llhttp (bundled C parser) is architecture-agnostic
• RISC-V hardware is shipping (SiFive, SpacemiT, Sophgo SG2044)

Fixes #136

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Note: this work is part of the RISE Project effort to improve Python ecosystem support on riscv64 platforms. Native riscv64 CI runners are available for free via RISE RISC-V runners.

[gounthar]

Native build verified on riscv64 hardware (BananaPi F3, SpacemiT K1, rv64gc, GCC 14.2.0): SUCCESS.

[gounthar]

For reference, a native riscv64 build on a RISE runner takes about 2 minutes: https://github.com/gounthar/httptools/actions/runs/23713272213

If QEMU build times become a concern, native runners are available via the RISE Project (https://github.com/apps/rise-risc-v-runners). But the QEMU approach in this PR works and avoids external dependencies.

[gounthar]

Updated docker/setup-qemu-action from v3 to v4 (pinned SHA ce360397).

[gounthar]

Friendly nudge on this one, no rush at all. I freshened the branch against current master and it is green; the new run is just waiting on a maintainer approval to start, since it is a fork PR. The riscv64 wheels build fine in the matrix.

Anything I can do to help move it along, or something you would rather I change first? Mostly I did not want it to quietly get lost in the pile.