The unittest test discovery right now only looks into sub-packages if they contain a `__init__.py` file. That's an unnecessary requirement, as packages are also importable if only `__init__.pyc` is present.
Sorry, I missed the little footnote on case 4 about still supporting source-less distributions. I guess in that context we do still need to support this. However - please check that this does indeed happen on Python master - 3.6. unittest does evolve and if you're testing on 2.7 or something your bug is perhaps already fixed.