RuntimeWarning: 'foo.__main__' found in sys.modules after import of package 'foo', but prior to execution of 'foo.__main__'

This warning appears in python3.5 and python3.6: see for example https://framagit.org/spalax/argdispatch/builds/7387.

/usr/lib/python3.5/runpy.py:125: RuntimeWarning: 'foo.__main__' found in sys.modules after import of package 'foo', but prior to execution of 'foo.__main__'; this may result in unpredictable behaviour
  warn(RuntimeWarning(msg))