Echo notifications

[andrew222651]

Description

An initial implementation of #1359. If you run listen channelname and the channel receives a notification, the next time you run a statement, the notification will be shown (similar to psql).

I'm not sure I'll have time to work on this further. If someone would like to take this and do what's necessary to get it merged (tests, confirm formatting, docs, ...) please do.

Checklist

• [ ] I've added this contribution to the changelog.rst.
• [ ] I've added my name to the AUTHORS file (or it's already there).

• [ ] I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• [x] Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)