pyproject.toml: PEP 639 compatibility

[adamjstewart]

Reverts #199
Fixes #200

PEP 621 is superseded by PEP 639. The previous syntax is deprecated and will be removed in a future version of setuptools.

@Bluefissure @EpicWink @mattip

[EpicWink]

You may want to add license-files = ["LICENSE"] to keep the license explicitly referenced (setuptools still includes the license file in the sdist and wheels anyway)

[adamjstewart]

The default value of license-files seems to include this. We can make it more explicit if we want.

[adamjstewart]

Two 👍, just waiting on a review from someone with merge privileges...

[adamjstewart]

@mattip is this ready to merge now?

[adamjstewart]

Trying to close out old PRs, is this still wanted?

[DimitriPapadopoulos]

I would like to see this PR merged.

[adamjstewart]

Would anyone with merge privileges still like to see this PR merged?

[mattip]

Truthfully, I don't know enough about the various setuptools versions. Will this break older versions?

[adamjstewart]

This setting requires setuptools 77+. By requiring this in pyproject.toml, pip will ensure that a suitable version is available in the build environment.

[EpicWink]

Truthfully, I don't know enough about the various setuptools versions. Will this break older versions?

Newer versions support newer specifications, and drop support for deprecated (parts of) specifications. In this case, sometime in the near future table-type license definitions will cause setuptools to raise an exception on build

Setuptools v77 requires Python 3.9+ (I can't remember about 3.8)

[DimitriPapadopoulos]

There's no point trying to keep compatibility with older versions. From the Python Packaging User Guide:

Ensure pip, setuptools, and wheel are up to date

While pip alone is sufficient to install from pre-built binary archives, up to date copies of the setuptools and wheel projects are useful to ensure you can also install from source archives:

python3 -m pip install --upgrade pip setuptools wheel

The only constraint is that cffi must support the latest version of setuptools that supports the earliest version of Python supported by cffi. As already stated, setuptools 77.0.3 does support Python 3.9.

[EpicWink]

... From the Python Packaging User Guide:

Ensure pip, setuptools, and wheel are up to date

While pip alone is sufficient to install from pre-built binary archives, up to date copies of the setuptools and wheel projects are useful to ensure you can also install from source archives:

python3 -m pip install --upgrade pip setuptools wheel

While not affecting any decisions in this pull request, note the the quoted section of the packaging user guide is out of date (and I'm in the process of removing: pypa/packaging.python.org#1999). The reality is that the latest compatible versions of build dependencies (including backends like setuptools, and their dependencies like wheel) are downloaded and used during sdist/wheel build.