sslutil: use raw strings for exception reason compare
Otherwise we attempt to compare a bytes to a str on Python 3
and it always fails.
Differential Revision: https://phab.mercurial-scm.org/D5721
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# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL