Use public .is_closed() instead of private ._check_closed() from asyncio

[shamrin]

trio-asyncio use private asyncio.BaseEventLoop._check_closed() method everywhere.

We may want to switch to public .is_closed() instead. It was introduced in Python 3.5.

See also: https://github.com/python-trio/trio-asyncio/pull/96#discussion_r551806302