Implement monitoring for the mypy wheel build

I think that at least two mypy releases have been delayed because the mypy wheel build (https://github.com/mypyc/mypy_mypyc-wheels) was broken and nobody noticed it for some time. It would be great if anybody interested in the health of the build could sign up to receive email notifications about build failures.

This blog post may be relevant (not sure though): https://ravgeetdhillon.medium.com/send-an-email-notification-when-github-actions-fails-ea83cbeabbe0

Steps:

1. Figure out a way of allowing people for subscribe to notifications (Google group?)
2. Implement email notifications
3. Break the build on purpose and check that a notification gets sent (both on master and in a release branch)
4. Unbreak the build

It would be great if somebody can look into this. If you need access to the wheels repo, please let me know and I can give you the access you'd need.

Beginnen Sie mit der Untersuchung des mypy_mypyc-wheels-Repositorys und seiner Build-Workflows, wobei Sie besonders auf master und Release-Branches achten. Ermitteln Sie, wie Abonnenten, E-Mail-Benachrichtigungen und Fehlerprüfungen behandelt würden, und überprüfen Sie anschließend das Ergebnis, indem Sie den Build absichtlich fehlschlagen lassen und bestätigen, dass Benachrichtigungen gesendet werden, bevor Sie ihn wiederherstellen.