Prepare NEWS for 1.3.1.dev1 release

This looks good, great summary!

A couple of things I noticed while reading:

Add sandbox API for command batching and use it for build, script, and compose elements.

This uses imperative voice, while the other entries don't. Maybe "There is a new sandbox API" would sound more natural.

Added Basic support for the BuildBox sandbox. The sand box will only be used if the environment variable BST_FORCE_SANDBOX is set to buildbox. This is the first step in transitioning to only using BuildBox for local sandboxing.

This change has also removed the need of a YAML cache as it proved to no longer be of benefit.

I don't see how an optional new sandbox backend could obsolete the YAML cache -- is this line in the correct place?

This change has also removed the need of a YAML cache as it proved to no longer be of benefit.

I don't see how an optional new sandbox backend could obsolete the YAML cache -- is this line in the correct place?

Ahh looks like this is just a muddle up, see: !1264 (diffs)

Thanks for the review. Good catch regarding the YAML cache point. It was a bad copy-pasta.

I've submitted !1602 (merged) to fix that. The same MR also changes the notes in imperative mood (Add X) to use past tense form (Added X). It's possible I may have missed some.