Release helper

[balloob]

Research what we can automate of this:

Validation steps:

• No open PRs in milestone for both dev and docs. All closed PRs have the cherry-picked label.
• No 404 links for links on the release blog post (easiest checked on https://rc.home-assistant.io preview website)

Execution steps:

• Gather breaking changes and add to release notes
• Merge website rc branch to current branch
• Cut new version for developers.home-assistant.io repo by running cd website && yarn version 0.73.0
• Update the environment variable pointing at blog post for the updater lambda
• push to PyPi
• Tweet