Make poll timeout patchable.

`select.select` ignores SIGINT on Windows. This is problematic with some unit tests with Tornado loops in threads. When I abort the execution with Ctrl+C it can happen that a thread is still alive and then I get an hour later (!) a stack trace with a KeyboardInterrupt happened in select. 

This can be easily fixed by patching the poll_timeout with a low value, but right now it is not really possible to do so (yes, you *could* hack `func_code.co_consts`, but I don't do this...).