build: Add the missing build-system section to fix building

[mgorny]

Add the missing build-system section to fix compatibility with strict PEP 517 installers. Per PEP 517, projects are required to either specify a backend explicitly or (for compatibility with legacy projects) provide a setup.py file.

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[woodruffw]

LGTM, thanks!

Per PEP 517, projects are required to either specify a backend explicitly or (for compatibility with legacy projects) provide a setup.py file.

FWIW, this isn't my read of the spec: PEP 517 says:

Where the build-backend key exists, this takes precedence and the source tree follows the format and conventions of the specified backend (as such no setup.py is needed unless the backend requires it). Projects may still wish to include a setup.py for compatibility with tools that do not use this spec.

In other words: setup.py is not needed for strict compatibility, but for installers that don't follow PEP 517 yet. My understanding of PEP 517 is that the absence of a build-backend when pyproject.toml is present means that the installer is expected to invoke setuptools.build_meta:__legacy__.

(However, I recognize the spec could be read both ways, and I don't really feel 100% confident about the above. Besides, your change is much more explicit anyways 🙂)

[woodruffw]

CI failure due to PR permissions, NFC.

[mgorny]

Thanks.