tests: fix get-with-headers.py on python3 when writing to stdout
Differential Revision: https://phab.mercurial-scm.org/D2306
FROM debian:__CODENAME__
RUN apt-get update && apt-get install -y \
build-essential \
debhelper \
devscripts \
dh-python \
less \
python \
python-all-dev \
python-docutils \
unzip \
zip