Trusted publishing: Support for CircleCI

[di]

Per https://circleci.com/docs/api/v2/index.html#tag/OIDC-Token-Management, CircleCI now supports a customizable aud claim which means we could support it as a trusted publisher.

This is likely blocked on https://github.com/pypi/warehouse/issues/13887 given that OIDC tokens from CircleCI might be intentionally shared across multiple third party providers.