opentimelineio: add build-opentimelineio.yml for riscv64 wheels - #1784

• Package: opentimelineio
• Version: 0.18.1
• Source: https://github.com/AcademySoftwareFoundation/OpenTimelineIO
• Docs: http://opentimeline.io

Compiles a pybind11 extension over a CMake C++ core (opentime/opentimelineio) with vendored Imath/rapidjson/pybind11 git submodules. Upstream publishes no riscv64 wheel.

Mirrors upstream's python-package.yml (package_sdist/package_wheels jobs).

Differs from upstream

• Stages Imath/rapidjson/pybind11 LICENSE files at the root - their code is statically compiled in but upstream's wheel omits the licences.
• Sdist built once on ubuntu-latest instead of per-arch, since it's architecture-independent.

Testing

• CIBW_TEST_SOURCES: tests examples, mirrors upstream's unittest discover suite plus the example script it shells out to.
• OTIO_DISABLE_SERIALIZED_SCHEMA_TEST=1 - docs/ is pruned from the sdist so the schema-doc diff test has nothing to compare against.

License: Wheel bundles Imath (BSD-3-Clause), rapidjson (MIT) and pybind11 (BSD-3-Clause) statically; upstream ships no licence text for them, so the build adds it.

Built on cp312; TBD passed, TBD skipped.