tests: update inline extensions in test-bundle2-exchange.t to py3
The test doesn't pass for what superficially look like good
reasons. We'll need to come back to it later.
# skip-blame because it's b prefixes and a couple of b'N' instead of str(N)
Differential Revision: https://phab.mercurial-scm.org/D3357
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF