I've updated the PEP to include:
- completion callbacks (for interoperability with Twisted Deferreds)
- a pointer to the discussion on stdlig-sig


Rejected ideas:
- Having a registration system for executors

Not yet addressed:
- where the package should live (someone in a "concurrent" package seems fine)
- having global executors with unbounded worker counts as a convenience [1]

[1] There are a few issues with global executors that need to be thought thought through i.e. when should workers be created and when should they be terminated. I'd be happy to defer this idea unless someone is passionate about it (in which case it would be great if they'd step in with concrete ideas).