Issue #20455: asyncio: write a new write pipe transport class for proactor (on

Issue #20455: asyncio: write a new write pipe transport class for proactor (on
Windows) instead of using the "duplex" pipe transport. The new class uses a
simpler overlapped read to be notified when the pipe is closed. So the protocol
doesn't need to implement eof_received(): connection_lost() is called instead.
_UnixWritePipeTransport has the same approach.