[3.7] bpo-32751: Wait for task cancellation in asyncio.wait_for() (GH-7216)

[miss-islington]

Currently, asyncio.wait_for(fut), upon reaching the timeout deadline,
cancels the future and returns immediately. This is problematic for
when fut is a Task, because it will be left running for an arbitrary
amount of time. This behavior is iself surprising and may lead to
related bugs such as the one described in bpo-33638:

condition = asyncio.Condition()
async with condition:
    await asyncio.wait_for(condition.wait(), timeout=0.5)

Currently, instead of raising a TimeoutError, the above code will fail
with RuntimeError: cannot wait on un-acquired lock, because
__aexit__ is reached before condition.wait() finishes its
cancellation and re-acquires the condition lock.

To resolve this, make wait_for await for the task cancellation.
The tradeoff here is that the timeout promise may be broken if the
task decides to handle its cancellation in a slow way. This represents
a behavior change and should probably not be back-patched to 3.6 and
earlier.
(cherry picked from commit e2b340a)

Co-authored-by: Elvis Pranskevichus elvis@magic.io

https://bugs.python.org/issue32751

[miss-islington]

@elprans and @1st1: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks!