tests/narrow-library.sh

py3: conditionalize initialization of stdio flags

Since Python 3 doesn't depend on the stdio of libc, there should be no need
to set O_BINARY flag on Windows.

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