add trio115: use trio.lowlevel.checkpoint() instead of trio.sleep(0)

[jakkdl]

I think 15 minutes to write a new check is a new record!

• I opted not to require it being in an async function or be await'ed, but can change that if you want
• feel free to suggest modifications to error message or documentation as usual
• did not update __version__ with there being multiple open PR's