snapcraft snap 失败，因为缺少 python 模块

我试图通过将 unity-tweak-tool 重新打包为 snap 来学习一些 snap 桌面打包。我发现文档不是很出色，但到目前为止，我有一个 snapcraft.yaml 文件，如下所示：

name: unity-tweak-tool
version: 1
summary: configuration tool for the Unity desktop environment
description: |
 Unity Tweak Tool is a settings manager for the Unity desktop.
 It provides users with a fast, simple and easy-to-use interface 
 with which to access many useful and little known features and settings 
 of the desktop environment that one may want to configure.

apps:
  unity-tweak-tool:
    command: $SNAP/usr/bin/unity-tweak-tool
    plugs: [unity7]
parts:
  unity-tweak-tool:
    plugin: python3
    source: git://github.com/Seth-Johnson/unity-tweak-tool.git
    source-type: git
    build-packages:
        - python3-distutils-extra
        - python3-all
        - python3-dbus
        - python-dev
    stage-packages:
        - gir1.2-glib-2.0
        - gir1.2-gtk-3.0
        - python3-xdg
        - python3-cairo
        - unity-webapps-service  

但是，当我运行snapcraft snapsetup.py 时总是失败，因为缺少几个 python 模块：

  Running setup.py install for unity-tweak-tool ... error
    Complete output from command /home/sethj/projects/snappy-testing/parts/unity-tweak-tool/install/usr/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-s6dwhgys-build/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-xxv1mi7p-record/install-record.txt --single-version-externally-managed --compile --home=/tmp/tmp2y5izah7:
    ERROR: Python module values not found
    ERROR: Python module apport not found
    ERROR: Python module apport.hookutils not found
    WARNING: syntax errors in notes/wizardry.py: Missing parentheses in call to 'print' (wizardry.py, line 9)
    ERROR: Python module dbus not found
    ERROR: Python module dbus.service not found
    ERROR: Python module dbus.mainloop.glib not found

（此处为完整输出）

我承认我不是 100% 确定我这样做是否正确，但据我所知，它应该构建 build-packages 下列出的包，它们满足 unity-tweak-tool 的构建要求。

我哪里错了？