It doesn't work when installed from a tarball either. After running with "--verbosity 9", I noticed this error:
Import of duplicity.backends.dpbxbackend Failed: cannot import name Dropbox
Running "pip install dropbox" showed that the Dropbox Python package was already installed:
Requirement already satisfied (use --upgrade to upgrade): dropbox in /usr/local/lib/python2.7/dist-packages
Requirement already satisfied (use --upgrade to upgrade): urllib3 in /usr/lib/python2.7/dist-packages (from dropbox)
However, I only had version 2.0.0 installed, and it looks like Duplicity requires a newer version now. I ran "pip install dropbox --upgrade" which upgraded it from 2.0.0 to 6.7.0, and now it's working.