Since this changes 32edb29 unittest.mock.Mock being detected
as asyncio.Future.
asyncio.futures.isfuture checks for private attribute _asyncio_future_blocking
to be not None and Mock() (without specs) returns new mock as this attribute.

The use case is here aio-libs-abandoned/aioredis-py#161 — basically, function wrapped with asyncio.coroutine
and set as Mock's method side_effect started to raise TypeError.