plyvel: add build-plyvel.yml for riscv64 wheels

[luhenry]

• Package: plyvel
• Version: 1.5.1
• Source: https://github.com/wbolster/plyvel
• Docs: https://github.com/wbolster/plyvel

Compiles plyvel's Cython/C++ bindings to LevelDB into a compiled extension, linking dynamically against LevelDB and Snappy built from source. Upstream publishes no riscv64 wheel.

Mirrors upstream's own Makefile release target and cibuildwheel-before-build.sh.

Differs from upstream

• Builds LevelDB and Snappy from source via cmake - upstream's prebuilt manylinux2014_x86_64 Docker image has no riscv64 equivalent.

Testing

• test/ staged apart from the checkout so pytest imports the installed wheel, not the source tree.
• Deselects test_open_read_only_dir - the test phase runs as root, which never sees the permission error it asserts.

License: Wheel dynamically links LevelDB and Snappy, both BSD-3-Clause like plyvel itself; the build adds their licence files since upstream's own wheels carry none.

Built on cp312; 53 passed, 1 deselected.