Produce stable artifacts

[jakirkham]

GitHub autogenerated source artifact from tags regularly have unstable checksums. As part of the release process it would be great if we could produce stable source artifacts (with checksums) and attach them to the release tag. This way we could more reliably consume the source in other places where we have builds (like conda-forge)

Here is a recent example: conda-forge/pynvml-feedstock#17 (comment)