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
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