Currently we use tornado to capture output from subprocesses in Launchers.

We can almost use the asyncio Subprocess machinery, but on Windows this is only supported by Proactor which requires Python 3.8. Additionally, Proactor doesn't support listening on FDs, which we also need for zmq sockets. So Subprocesses and FDs are mutually exclusive features on Windows unless you use tornado >= 6.1 with proactor instead of asyncio directly, which is just an unreasonably strict requirement.

So maybe requiring tornado is the right thing to do, but that means we need to be careful to ensure tornado is running when we need it to be.