import(..., fromlist=['']) isn't unique to Mitogen. It's also in imagepy, webpy, cherrypy, and others https://grep.app/search?q=. Most examples use the returned value. I think it's meant as a way to get the leaf module, rather than the top-level package, e.g.
>>> import('xml.dom.minidom')
<module 'xml' from '.../xml/init.py'>
>>> import('xml.dom.minidom', fromlist=[''])
<module 'xml.dom.minidom' from '.../xml/dom/minidom.py'>