Code review comment for lp://staging/~jelmer/loggerhead/drop-python3

ols-jenkaas is explicitly running "tox -e py27 -e py35". Dropping py27 will have to wait for the new year so that I can get somebody to update the job configuration.

However, there's also another problem that could be fixed before then:

14:40:30 O: py35 create: /workspace/loggerhead/loggerhead/work/.tox/py35
14:40:33 O: py35 installdeps: -rrequirements.txt
14:40:37 O: ERROR: invocation failed (exit code 1), logfile: /workspace/loggerhead/loggerhead/work/.tox/py35/log/py35-1.log
14:40:37 O: ERROR: actionid: py35
14:40:37 O: msg: getenv
14:40:37 O: cmdargs: [local('/workspace/loggerhead/loggerhead/work/.tox/py35/bin/pip'), 'install', '-rrequirements.txt']
14:40:37 O: env: {'_': '/usr/bin/tox', 'LOGNAME': 'ubuntu', 'no_proxy': '0.0.0.0,127.0.0.1,localhost,launchpad.net,ubuntu.com,canonical.com,dashboard.snapcraft.io,dashboard.staging.snapcraft.io', 'SSH_CONNECTION': '10.30.97.1 59316 10.30.97.224 22', 'USER': 'ubuntu', 'PATH': '/workspace/loggerhead/loggerhead/work/.tox/py35/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin', 'OLDPWD': '/home/ubuntu', 'MAIL': '/var/mail/ubuntu', 'http_proxy': 'http://squid.internal:3128/', 'VIRTUALENV_PIP': '20.3.4', 'XDG_RUNTIME_DIR': '/run/user/1000', 'HTTPS_PROXY': 'http://squid.internal:3128/', 'PYTHONHASHSEED': '102812890', 'SSH_CLIENT': '10.30.97.1 59316 22', 'https_proxy': 'http://squid.internal:3128/', 'BRZ_PLUGIN_PATH': '-user:-site', 'XDG_SESSION_ID': 'c3', 'LANG': 'en_US.UTF-8', 'NO_PROXY': '0.0.0.0,127.0.0.1,localhost,launchpad.net,ubuntu.com,canonical.com,dashboard.snapcraft.io,dashboard.staging.snapcraft.io', 'BRZ_PLUGINS_AT': 'loggerhead@/workspace/loggerhead/loggerhead/work', 'VIRTUALENV_DOWNLOAD': '0', 'HOME': '/home/ubuntu', 'SHELL': '/bin/bash', 'PWD': '/workspace/loggerhead/loggerhead/work', 'SHLVL': '1', 'VIRTUAL_ENV': '/workspace/loggerhead/loggerhead/work/.tox/py35', 'HTTP_PROXY': 'http://squid.internal:3128/'}
14:40:37 O:
14:40:37 O: Collecting https://www.owlfish.com/software/simpleTAL/downloads/SimpleTAL-5.2.tar.gz (from -r requirements.txt (line 7))
14:40:37 O: Downloading https://www.owlfish.com/software/simpleTAL/downloads/SimpleTAL-5.2.tar.gz (96kB)
14:40:37 O: Requirement already satisfied (use --upgrade to upgrade): setuptools in ./.tox/py35/lib/python3.5/site-packages (from -r requirements.txt (line 1))
14:40:37 O: Collecting Paste>=1.6 (from -r requirements.txt (line 2))
14:40:37 O: Downloading https://files.pythonhosted.org/packages/7a/b0/ca8825989b9c6203f36f80fef31be5144df3ff3461efec3f34615b6a7f99/Paste-3.5.0-py2.py3-none-any.whl (593kB)
14:40:37 O: Collecting dulwich (from -r requirements.txt (line 3))
14:40:37 O: Downloading https://files.pythonhosted.org/packages/c7/f0/04e7aae32c5c45666fa94fa146ba3a6f78d44ed9fd11f3fc497bd6a3d13e/dulwich-0.20.26.tar.gz (416kB)
14:40:37 O: Complete output from command python setup.py egg_info:
14:40:37 O: Traceback (most recent call last):
14:40:37 O: File "<string>", line 1, in <module>
14:40:37 O: File "/tmp/pip-build-2uby56gq/dulwich/setup.py", line 22, in <module>
14:40:37 O: 'Dulwich only supports Python 3.6 and later. '
14:40:37 O: Exception: Dulwich only supports Python 3.6 and later. For 2.7 support, please install a version prior to 0.20

This looks like dulwich needs to be pinned for `python_version < "3.6"`?