Add pyproject.toml and support Cython 3

[dalcinl]

No description provided.

[benlandrum]

Looks like you have the rest of this. Is the MacOS ARM issue new? I don't remember seeing that one previously.

[dalcinl]

Looks like you have the rest of this

I still have to fix Azure Pipelines. I'll take a look tomorrow morning. I'm not sure whether to keep insisting with conda-forge environments, of just use the same approach as here, but for the older Python versions (like I do in mpi4py).

Is the MacOS ARM issue new?

The ARM macOS images do not have the older versions of Python. Other than that, there are no issues. What do you meant exactly?

[benlandrum]

The ARM macOS images do not have the older versions of Python. Other than that, there are no issues. What do you meant exactly?

Yes, that's the issue. I had for instance macos-latest and Python 3.7 working a few weeks ago with my draft, when macos-latest was 12.7.4 (see here). Now on 14.4.1 I get The version '3.7' with architecture 'arm64' was not found for macOS 14.4.1.

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[dalcinl]

For the time being, testing of all he IO stuff will not run and is uncovered.
The build fixes and Cython 3 updates are more important that 100% test coverage.