Following #999, the idea here was to use non-blocking sockets and catch
EINTR then use this event to detect when the OS is blocking on writes.
The problem is that python3 actually retries syscalls automatically on
EINTR so that's a non-starter.
There may be other socket options we can use on Linux: socket man page but this won't be portable.
The only way to do this would be to use native OS calls for the network layer.
For Linux, TCP_INFO: #2421.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2156