Replace setuptools/bundled-capnproto build with conan-py-build + CMake [looking for feedback]

[czoido]

Hi from the Conan team!

We recently released conan-py-build, a PEP 517 build backend that uses Conan to manage C/C++ dependencies, and we're testing it on several popular Python extensions that we think could benefit from it.

Here's the launch blog post: https://blog.conan.io/cpp/conan/python/2026/05/05/Introducing-conan-py-build.html
And the repo and docs: https://github.com/conan-io/conan-py-build · https://conan-py-build.conan.io/

We wanted to share this port of pycapnp to conan-py-build to get some comments and feedback from the project. The capnproto C++ library is fetched from Conan Center Index instead of being downloaded as a tarball from capnproto.org at build time.

This is a draft PR for sharing the work to get your thoughts on whether this direction is interesting to the project.

Some notes about the migration:

• No bundled tarball. setup.py previously downloaded capnproto-c++-1.0.1.tar.gz from capnproto.org at build time. Conan now fetches and builds capnproto/1.0.1 from Conan Center Index.
• Build tools managed by Conan. Building capnproto requires autotools (m4, etc.) to be present on the system. Conan manages these build tools as well.
• Explicit, versioned C++ dependency. Bumping the C++ library is a one-line change in conanfile.py.
• setup.py + buildutils/ removed. ~350 lines of build machinery replaced by a minimal conanfile.py and a standard CMakeLists.txt.
• capnp/version.py simplified. Previously generated dynamically by setup.py; now it's a static file using importlib.metadata.
• Same CI workflow. cibuildwheel keeps driving multi-platform builds with minimal changes.
• Equivalent wheels. We compared the output against the 2.2.3 PyPI release file-by-file across all platforms. The only differences are cosmetic metadata.

CI results: https://github.com/czoido/pycapnp/actions/runs/27526912439

Any feedback is very welcome!

Thanks a lot 🙂