Petr, this could be an interesting issue to take a look at from the point of view of the builtin and extension module import changes in Python 3.5.

Given the extent of the import system changes across 3.3/4/5, it's even conceivable we may have made this work somewhere along the line. In that case we'd still need a new regression test to ensure it keeps working - we didn't even have a regression test to ensure extension module imports from inside a package worked, so we definitely don't have one for builtin submodules.