Issue #21326: Add a new is_closed() method to asyncio.BaseEventLoop

Add BaseEventLoop._closed attribute and use it to check if the event loop was
closed or not, instead of checking different attributes in each subclass of
BaseEventLoop.

run_forever() and run_until_complete() methods now raise a RuntimeError('Event loop is
closed') exception if the event loop was closed.

BaseProactorEventLoop.close() now also cancels "accept futures".