Issue 35409: Async generator might re-throw GeneratorExit on aclose()

As far as I can tell, this issue is different than: https://bugs.python.org/issue34730

I noticed `async_gen.aclose()` raises a GeneratorExit exception if the async generator finalization awaits and silence a failing unfinished future (see example.py).

This seems to be related to a bug in `async_gen_athrow_throw`. In fact, `async_gen.aclose().throw(exc)` does not silence GeneratorExit exceptions. This behavior can be reproduced without asyncio (see test.py).

Attached is a possible patch, although I'm not too comfortable messing with the python C internals. I can make a PR if necessary.

New changeset 8e0de2a4808d7c2f4adedabff89ee64e0338790a by Miss Islington (bot) (Vincent Michel) in branch 'master':
bpo-35409: Ignore GeneratorExit in async_gen_athrow_throw (GH-14755)
https://github.com/python/cpython/commit/8e0de2a4808d7c2f4adedabff89ee64e0338790a

New changeset 6c3b471c8c0bfd49c664d8ee7e95da3710fd6069 by Miss Islington (bot) in branch '3.8':
bpo-35409: Ignore GeneratorExit in async_gen_athrow_throw (GH-14755)
https://github.com/python/cpython/commit/6c3b471c8c0bfd49c664d8ee7e95da3710fd6069

New changeset 4ffc569b47bef9f95e443f3c56f7e7e32cb440c0 by Miss Islington (bot) in branch '3.7':
bpo-35409: Ignore GeneratorExit in async_gen_athrow_throw (GH-14755)
https://github.com/python/cpython/commit/4ffc569b47bef9f95e443f3c56f7e7e32cb440c0

I'm not sure this was the correct fix - or at least, this creates further issues with asynccontextmanager. Consider the following code:

---------------
import contextlib
import types

@contextlib.asynccontextmanager
async def acm():
    # GeneratorExit athrown here from AsyncContextManager __aexit__,
    # propagated from the body of the contextmanager in func()
    yield

@types.coroutine
def _yield():
    yield

async def func():
    async with acm():
        # GeneratorExit raised here
        await _yield()

x = func()
x.send(None) # start running func
x.close() # raise GeneratorExit in func at its current yield
# AsyncContextManager __aexit__ fails with "RuntimeError: generator didn't stop after throw()"
---------------

The reason for the failure in AsyncContextManager __aexit__ is that the asyncgenerator raises StopIteration instead of GeneratorExit when agen.athrow(GeneratorExit()) is called and driven, so "await agen.athrow(GeneratorExit())" just evaluates to None, rather than raising GeneratorExit.

On 3.6 this would work fine, because "await athrow(GeneratorExit())" will raise GeneratorExit. I suspect this was broken by this change.

My mistake, I see now this is just https://bugs.python.org/issue33786 and is already fixed.