Add a build for the source distribution

[jnothman]

It seems silly that:

• the nightly builds do not include an sdist (to avoid users having to cythonize)
• the release maintainer needs to build the sdist separate from the wheels.

We should have a job here that uploads an sdist alongside the wheels... as long as that then works with fetch_artifacts.

[rth]

+1.

Do we still want to cythonize in sdists though? It's seems like Cython generated .c files is not forward compatible with future Python versions, and if I remember correctly there were several issues reported about it. Adding cython to build requirements (or what goes for it pre PEP 518) could be a workaround.

[ogrisel]

Good remark for Cython but this is a separate issue: we still have it if we do the sdist manually.

[ogrisel]

+1 for automating the generation of the sdist archive. That would also give us the ability to test installation from the source tarball. We could also run twine check on it.

[jnothman]

Yes, we've messed up sdist generation before.