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
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL