Fix cffi extension build configuration

[dataflake]

Closes #222

CFFI does not support pyproject.toml yet, instructions and prerequisites for building CFFI extension modules must stay in setup.py for now.

[davisagli]

Maybe instead it would make sense to update https://github.com/zopefoundation/persistent/blob/master/.github/workflows/tests.yml to use https://build.pypa.io/en/stable/ (which uses pyproject.toml) instead of calling setup.py directly? That is what is documented here: https://packaging.python.org/en/latest/tutorials/packaging-projects/#generating-distribution-archives

I'm okay with this as a quick interim fix though!

[dataflake]

If someone wants to research how CFFI extensions can be built without specifying them in setup.py as cffi_modules argument please go ahead. The CFFI documentation I have seen says that's still the only supported way.

[davisagli]

@dataflake okay, I see now. Here's an open issue in cffi about it: python-cffi/cffi#55

[dataflake]

@icemac I have released version 6.3 and tests are green on Zope and other packages that failed before, like Persistence. persistent release 6.2 is essentially a brown-bag release, the wheels for macOS and Windows are broken due to the missing extension file. That was my fault, I removed setup_requires earlier.

Do you agree that I should yank release 6.2 on PyPI?

[icemac]

@dataflake wrote:

Do you agree that I should yank release 6.2 on PyPI?

Thank you for fixing this issue. Yes, please yank 6.2 on PyPI to signal people not to use it.

[dataflake]

Release 6.2 is now yanked from PyPI.