Set release schedule for 20.1

It's that time again, time to start talking about the next release. I've provided a proposed calendar (based on the not-yet-merged 20.0.1 release notes, so I don't have to rebase later) for 20.1. It's possible that I'll be doing this relase, or it's possible that @eric will be helping, so I've left the maintainer as TBD for now.

Is everyone okay with the proposed dates?