We just released a preview image for Xcode 9.0 beta 1, running on Sierra, with the latest Simulators.
You can use this image by specifying Xcode 9.0 in your circle.yml:
machine: xcode: version: "9.0"
This image tag will be automatically upgraded when new betas are released by Apple, until the final release of Xcode 9.
You can find the full list of changes for this release on the image tests repo: https://github.com/circleci/macos-image-tests/pull/9