Nit: replace Engine._event_handlers dict by defaultdict

[vfdev-5]

I was just puzzled for some seconds by the code:

if event_name not in self._event_handlers:
    self._event_handlers[event_name] = []

and thus for ease of understanding it could be replaced by defaultdict.
This PR is really a code polishing, so if there are some objections, we can easily close it.
What do you guys think ?

[alykhantejani]

LGTM, not sure why I didn't use a defaultdict in the first place.