Cannot build ubuntu-image snap

It looks like we can no longer build the u-i snap from its git repo snapcraft.yaml file. I don't yet know whether this is caused by a bug in snapcraft, or we need to update the yaml file for recent Python plugin changes. It obviously worked at some point (and we've been using python3-debian to generate the version.txt for quite a while now).

Successfully installed pip-8.1.2 setuptools-26.1.1 wheel-0.29.0
pip download --disable-pip-version-check --dest /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages python-debian attrs voluptuous PyYAML
Collecting python-debian
  Using cached python-debian-0.1.28.tar.gz
  Saved /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages/python-debian-0.1.28.tar.gz
Collecting attrs
  Using cached attrs-16.2.0-py2.py3-none-any.whl
  Saved /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages/attrs-16.2.0-py2.py3-none-any.whl
Collecting voluptuous
  Using cached voluptuous-0.9.3.tar.gz
  Saved /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages/voluptuous-0.9.3.tar.gz
Collecting PyYAML
  Using cached PyYAML-3.12.tar.gz
  Saved /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages/PyYAML-3.12.tar.gz
Collecting six (from python-debian)
  Using cached six-1.10.0-py2.py3-none-any.whl
  Saved /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages/six-1.10.0-py2.py3-none-any.whl
Collecting setuptools>=0.6b1 (from voluptuous)
  File was already downloaded /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages/setuptools-28.3.0-py2.py3-none-any.whl
Successfully downloaded python-debian attrs voluptuous PyYAML six setuptools
pip download --disable-pip-version-check --dest /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages .
Processing /home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/src
  Link is a directory, ignoring download_dir
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-v38gpdm9-build/setup.py", line 5, in <module>
        from debian.changelog import Changelog
    ImportError: No module named 'debian.changelog'

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-v38gpdm9-build/
Command '['/bin/sh', '/tmp/tmph7epnkp2', 'pip', 'download', '--disable-pip-version-check', '--dest', '/home/barry/projects/ubuntu/allsnappy/ubuntu-image/parts/ubuntu-image/packages', '.']' returned non-zero exit status 1