Google Code-in

Help add mypy annotations to the Zulip codebase! This is a great way to practice git, git grep, and efficiently using a powerful editor.

Instructions for mypy annotation tasks are at https://github.com/zulip/zulip-gci/blob/master/tasks/mypy-annotations.md.

For this task, do Task Type B for the following file: bots/gcal-bot.