Added the psycopg event broker

Changes

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that you've fulfilled the following conditions (where applicable):

[x] You've added tests (in tests/) added which would fail without your patch
[x] You've updated the documentation (in docs/, in case of behavior changes or new features)
[x] You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version. If, say, your patch fixes issue #999, the entry should look like this:

* Fix big bad boo-boo in the async scheduler (#999 <https://github.com/agronholm/apscheduler/issues/999>_; PR by Yourname)

If there's no issue linked, just link to your pull request instead by updating the changelog after you've created the PR.

If possible, use your real name in the changelog entry. If not, use your GitHub username.

agronholm / apscheduler