The stub for asyncio.as_completed declared it as taking a Sequence of
Futures. This was unnecessarily strict.
Just like asyncio.wait, asyncio.as_completed can be declared to take
an Iterable of Futures instead. Both these functions iterate over
their argument only once to store its items in a set, so an Iterable
is sufficiently strict. This has been true since the initial
implementation of the functions.
