Fails reproducible on hydra like this:
```
try:
# set_wakeup_fd() raises ValueError if this is not the
# main thread. By calling it early we ensure that an
# event loop running in another thread cannot add a signal
# handler.
> signal.set_wakeup_fd(self._csock.fileno())
E ValueError: set_wakeup_fd only works in main thread of the main interpreter
```
And was remove from upstreams tests due to 10-15% error rate way back
in 2018.
af84578114