Issue 39606: Regression: it should be possible to close async iterators multiple times

In bpo-39386, the 'aclose' method for async generators was fixed so that the following broken code would correctly raise an error:

# -- bad code --
async def agen_fn():
    yield

async def do_bad_thing():
    agen = agen_fn()
    aclose_coro = agen.aclose()
    await aclose_coro
    # Should raise RuntimeError:
    await aclose_coro

asyncio.run(do_bad_thing())

# -----------------

However, the merged fix also broke the following correct code, which should *not* raise an error:

# -- good code --
async def agen_fn():
    yield

async def do_good_thing():
    agen = agen_fn()
    await agen.aclose()
    # Should *not* raise an error, but currently does in Python dev branches:
    await agen.aclose()

asyncio.run(do_good_thing())

# ----------------

It's not supported to iterate the same coroutine object twice. However, making two *independent* calls to aclose should return two independent coroutine objects, and it should be legal to iterate over each object once.

This can also occur even if there's only a single call to 'aclose'. For example, this is the recommended idiom for making sure that an async generator correctly closes all its resources:

# -- good code --
async def agen_fn():
    yield

async def careful_loop():
    agen = agen_fn()
    try:
        async for _ in agen:
            pass
    finally:
        await agen.aclose()

asyncio.run(careful_loop())

# -------------------

On released Python, the code above runs correctly. On the latest Python dev branches, it raises a RuntimeError.

So basically the problem is that the fix for bpo-39386 is storing the "was aclose iterated?" state on the async generator object, but in fact it needs to be stored on the aclose coroutine object.

New changeset 925dc7fb1d0db85dc137afa4cd14211bf0d67414 by Nathaniel J. Smith in branch 'master':
bpo-39606: allow closing async generators that are already closed (GH-18475)
https://github.com/python/cpython/commit/925dc7fb1d0db85dc137afa4cd14211bf0d67414

New changeset 8dbdf5f275c6462bb522bcf3a29054239d72989d by Miss Islington (bot) in branch '3.8':
[3.8] bpo-39606: allow closing async generators that are already closed (GH-18475) (GH-18501)
https://github.com/python/cpython/commit/8dbdf5f275c6462bb522bcf3a29054239d72989d

New changeset f464edf3239f7867fe31c9cd238a68fb3b90feaa by Nathaniel J. Smith in branch '3.7':
bpo-39606: allow closing async generators that are already closed (GH-18475) (GH-18502)
https://github.com/python/cpython/commit/f464edf3239f7867fe31c9cd238a68fb3b90feaa