Issue 28430: asyncio: C implemeted Future cause Tornado test fail

https://travis-ci.org/tornadoweb/tornado/jobs/167252979

Another test failure is reported.

> I have setup a 3.6 build on Travis of our project GNS3 and it seem to fail.
> https://travis-ci.org/GNS3/gns3-server/builds/167703118

pure Python Future.__iter__ don't use what yield-ed.

    def __iter__(self):
        if not self.done():
            self._asyncio_future_blocking = True
            yield self  # This tells Task to wait for completion.
        assert self.done(), "yield from wasn't used with future"
        return self.result()  # May raise too.

I felt no-None value is sent by iter.send(val) wasn't make sense.
But Tornado did it (maybe, for compatibility to Tornado's generator.)

So this patch ignores when non-None value is passed.
Additionally, I moved NEWS entry about C Future from "core and builtin"
section to "library" section.

Test failure in GNS3 seems not relating to this.
It may be fixed via #28492, maybe.

The patch looks good.  2 things:

- It appears it also touches Misc/NEWS a bit too much. Please make sure to not to commit that.

- I'd also add a comment explaining why we ignore values passed to FI.send() and simply send None.  The reason is how Future.__iter__ is designed:

   class Future:
      def __iter__(self):
        if not self.done():
            self._asyncio_future_blocking = True
            yield self  # This tells Task to wait for completion.
        assert self.done(), "yield from wasn't used with future"
        return self.result()  # May raise too.

^-- Future.__iter__ doesn't care about values that are pushed to the generator, it just returns "self.result()".

New changeset b471447352ac by INADA Naoki in branch '3.6':
Issue #28430: Fix iterator of C implemented asyncio.Future doesn't
https://hg.python.org/cpython/rev/b471447352ac

New changeset bd141ec2973a by INADA Naoki in branch 'default':
Issue #28430: Fix iterator of C implemented asyncio.Future doesn't
https://hg.python.org/cpython/rev/bd141ec2973a

> - It appears it also touches Misc/NEWS a bit too much. Please make sure to not to commit that.

I wont to move move this entry from "Core and Builtins" section to "Library" section:

- Issue #26081: Added C implementation of asyncio.Future.
  Original patch by Yury Selivanov.

May I do it?

+- Issue #28430: Fix iterator of C implemented asyncio.Future doesn't accept

+  non-None value is passed to it.send(val).

This NEWS entry is not a coherent English sentence.  Please revise.  I don't understand it well enough to suggest a revision, but would review a replacement.

I'm sorry about my bad English.

> Fix iterator of C implemented asyncio.Future

This meant:

fut = asyncio.Future()  # C implemented version of asyncio.Future
it = iter(fut)  # Iterator of it
it.send(42)     # raised TypeError before. It was not compatible with Python version of asyncio.Future

stage: commit review -> resolved

stage: commit review