Publish eth2spec package again

[jtraglia]

We used to publish the eth2spec package to PyPI but we stopped mid-2022. I'm not entirely sure why we stopped, but it would be useful to have this. Before doing this, we might want to rename eth2spec to something else. I'll need to think about this more.

• Researchers can quickly prototype things outside of consensus-specs repo.
• Developers (eg https://github.com/serenita-org/vero) can use types & functions in tooling.
• We can use eth2spec to create a slow consensus layer client for testing.