See #2

We moved to SHA256 for compatibility with older versions of Python 3, and this hash is thus unchecked. It will only get out of date, so it seems better to remove it.

This means we must downgrade to SHA256, as SHA3 is not in hashlib.

Separating this into principal and auxiliary submitters was
chosen by NIST. If we do not do the same thing, we would have to
decide how to order names. People seem to have opinions on ordering.