Update void elements

[annevk]

https://github.com/html5lib/html5lib-python/blob/master/html5lib/constants.py#L487 is no longer accurate. Seems to miss keygen, wbr, menuitem.

And seems to have event-source which is no longer a thing.

[jayaddison]

@willkg @gsnedders fyi - it looks like #395 can contribute to resolving this issue.

NB: The code block referenced in the issue description within constants.py is now viewable here.

[ambv]

Looks like menuitem is deprecated, and moreover not really void.

keygen is also deprecated.

So the only missing one is wbr.

I don't think it's worth adding keygen as it's already deprecated and was never used in html5lib. Moreover, I think we could rather uncontroversially remove command (removed entirely from the spec in 2012), and event-source (renamed to eventsource in July 2008 and dropped entirely in February 2009).