There's no 3.12.1 sdist on PyPI

[Luiz Guilherme Fonseca Rosa]

Hi, fellow developers.

I don't know if you give attention to GitHub issues, but there is one (https://github.com/protocolbuffers/protobuf/issues/7520) opened 6 days ago that is highly blocking.

For example, one of my projects use Newrelic, which doesn't pin Protobuf a specific version, but "<4". It is failing to lock and install dependencies because it fails to download 3.12.1, which is the latest version.

It is blocking builds on multiple projects. Could you please take a time to fix it? I don't think it should take more than a few minutes.

Thanks,

@luizguilhermefr