tests/unwrap-message-id.py

py3: add extra traceback line present on Python 3.8

I'm not sure why Python 3.8 is outputting this line. It
appears to be a change in behavior of formatting tracebacks on
Python 3.8. So let's add it to expected output.

With this change, test-hook.t now passes on Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D7946

from __future__ import absolute_import, print_function

import re
import sys

print(re.sub(r"(?<=Message-Id:) \n ", " ", sys.stdin.read()), end="")