Improve KeyboardInterrupt handling on Windows

the ProactorEventLoop used by asyncio on Windows does not implement add_signal_handler
- on UNIX, we use it to catch sigint early, print a message and cancel the task that runs the AutoscaledPool
- without the custom signal handler, the code can be cancelled basically anywhere -> some worker tasks may be ended prematurely
it should be possible to register a signal handler with signal.signal, but cancelling the right task from there may be tricky

resolved by #2277

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