Python 3.8

[minrk]

Issue to track adding Python 3.8 to the images, which it looks like should happen at the end of July, with 3.8.0b3.

From the release notes for 3.8b1, 3.8 ABI is planned to be frozen in beta 3, which is scheduled for 2019-07-29.

Or maybe the policy in this repo should be freezing the ABI with release candidates? That would give a much shorter window for developers to start publishing wheels with this tool (3 weeks vs 3 months).

[fikisipi]

I think beta previews should be part of the images. Wheels should be built before the releases happen.