Automate Zulip release announcement

[abelsromero]

What is this issue about?

• Bug report
• Feature request
• Question

Description

Zulip's announce stream is used to announce a release once is done.
Once figured out how to obtain a key, seems that posting can be made quite easily with curl https://zulip.com/api/send-message.

[abelsromero]

Moving to next minor milestone, I don't want to delay the release and I realized we need to fully automate the whole release (there's still a manual step to "close the repo").