Consider how to handle ABI dependencies of Python dependencies

[qwcode]

It would be great if projects (Wheels Mostly? Possibly installed projects too?) could declare their ABI version and then other projects could declare a dependency on a particular ABI as well. Consider the case of CFFI where a project working with CFFI can more or less work with any version of CFFI's API, but once you compile the CFFI module it depends on the ABI of the exact version of CFFI installed. I don't know how this would look exactly though.

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migrated from https://bitbucket.org/pypa/pypi-metadata-formats/issues/50/consider-how-to-handle-abi-dependencies-of