Specify cffi_modules via pyproject.toml

Created originally on Bitbucket by Anonymous

For setuptools-less packages (or setuptools ones without a setup.py, which post PEP-517/PEP-518 is now a thing), it'd be nice to be able to specify cffi_modules without needing to have a setup.py just for CFFI.

See e.g. how this works for setuptools-scm: https://github.com/pypa/setuptools_scm/#pyprojecttoml-usage.

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