sslutil: migrate to hashutil.sha1 instead of hashlib.sha1 This is a straight-line replacement like the others, but I split it out since it's used in a network context and I'm not sure this is appropriate (we should probably drop support for sha1 fingerprints over TLS) and wanted this to be easily dropped. Differential Revision: https://phab.mercurial-scm.org/D7850