In case it is useful for reference, I beat my head on it a bit more and replaced "main" with "9.6": deb http://apt.postgresql.org/pub/repos/apt/ xenial-pgdg 9.6
Yes, but to quote Douglass Adams, "It was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying ‘Beware of the Leopard'."
If I might be so bold, one or more of the following would make life easier on users and perhaps get more people to test:
By way of example, from the snapshots page clicking on the link under Red Hat (and variants) takes you to http://yum.postgresql.org/ page where 9.6 beta is right at the top. From there all you have to do is select your release of choice.
By comparison, follow the https://apt.postgresql.org/ link under Debian, Ubuntu Linux and you are redirected to the wiki where, if you are fortunate, you will find "Have a look at the FAQ." nestled about half-way down then dig through that page. I never got there but figured it out by clicking through the file browser after wasting time seeing lots of 9.6 utilities/addons already listed but broken due to the lack of availability of the actual server and client.
I know this is open source. I know that people work on their "itch" or what their employer sponsors. I'm just sharing the user experience should it provide value and increase the number of testers.