tests/narrow-library.sh

py3: use stringutil.forcebytestr() to convert error messages to bytes

Differential Revision: https://phab.mercurial-scm.org/D3284

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF