This is a minor issue, so let's leave it for someone who hasn't made any PRs yet and wants a simple one to practice one.

The issue: lots of Trio's modules have __all__ variables defined. That's because once upon a time, trio/__init__.py used these __all__ variables to figure out what to re-export as part of the main namespace. But we stopped doing that a long time ago, because it confused static analysis tools. So now the __all__ variables are vestigial, aren't being updated, and don't actually do anything except clutter up the code, and potentially confuse readers. So we should do a sweep through and delete them all.