Even if canercandan doesn't give more information, I now understand that python setup.py install doesn't install dependencies, only the main package. This is unlike easy_install allmydata-tahoe and pip install allmydata-tahoe and unlike just doing python setup.py build and running the software from the current directory, as is recommended in quickstart.rst. All of those three options will automatically resolve dependencies. See, for example, sel's recent bug report: #2063.
I'm leaving this ticket open. To fix it, switch our build system to a tool which automatically resolves dependencies when you run python setup.py install. (Note: I'm not aware of any build tool that already does this for you, so I guess you would have to create a bespoke tool for Tahoe-LAFS or else contribute a patch to some tool like setuptools and then use it in Tahoe-LAFS.)
