Synchronize publishing of release artifact files

There are some timing holes in the current cPython release process which, while somewhat minor, could be tightened up. Currently, there are three release team member roles in producing downloable artifacts for a release and each is responsible for ensuring that their artifacts have been moved into the final file system location on the download server. Once they have done that, those subsets of the release artifacts become immediately available to download, independent of any release notice. Since the three release team members in general work asynchronously once the RM tags the release engineering branch, it is unpredictable when various release files appear on the download server. The release automation waits at various points for the artificats to show up there, also independent of any manual (email) notification to the RM.

It is up to the RM to finish things off and produce the sigstore files, update the release database (addfiles), and announce the release. But because the URLs of the downloadable files for each release are generally predictable, there are some downstream users and likely bots that can poll for these files becoming available. This can, on infrequent occasions, be an issue if some problem is discovered late in the release process, for example, a problem that is discovered while testing the macOS installer or the Windows binaries, that may require a re-spin of the entire release.

Granted users shouldn't be using things before they are officially announced, but they will. It would be better if we modified the process somewhat so the RM is responsible for triggering the final step of making all of the downloadable artifacts publically available after all the pieces are in place.

首先梳理 issue 中描述的 release automation 和 release team 的三个角色，包括与 download server 之间的交接。检查 RM 生成 sigstore 文件、使用 addfiles 更新 release database 以及发布 release 公告的步骤。完成的标准是：每个 artifact 都准备就绪后，RM 可以触发一次最终的 publication step。