Add idempotent close method

[jvansanten]

AioPipeStream.__del__() relies on os.close() raising OSError upon attempts to double-close a file handle. If someone calls os.pipe() between AioPipeStream.detach() and AioPipeStream.__del__(), however, a new file handle may be created with the same value, causing AioPipeStream.__del__() to happily close a different pipe that may still be in use. This can happen when multiple subprocesses are in flight at once. This PR adds close() method that explicitly invalidates the held file handle, avoiding random double closes.