ci: add riscv64 wheels

[justeph]

What do these changes do?

Now that cibuildwheel and PyPI support riscv64, we can start building riscv64 wheels.

Are there changes in behavior for the user?

multidict now runs on riscv64 machines :)

Related issue number

No issue

Checklist

• I think the code is well written
• Documentation reflects the changes

[justeph]

FYI, RISE has been building and distributing multidict manylinux wheels for riscv64 for several months (versions 6.7.1, 6.7.0, 6.6.4, 6.6.3, 6.6.2, 6.5.1, 6.4.4, 6.4.3 6.4.2, 6.2.0, 6.1.0). Tested builds are available for riscv64 here.

[codspeed-hq]

Merging this PR will not alter performance

✅ 245 untouched benchmarks

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_{Comparing justeph:ci-add-riscv64-support (0044178) with master (240dab7)}