Fix asynchronous generators athrow() and aclose() to handle StopAsyncIteration

[1st1]

BPO	28721
Nosy	@ned-deily, @1st1
PRs	[Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	agen.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/1st1'
closed_at = <Date 2016-11-16.23:26:54.874>
created_at = <Date 2016-11-16.23:12:03.626>
labels = ['interpreter-core', 'type-bug', '3.7', 'release-blocker']
title = 'Fix asynchronous generators athrow() and aclose() to handle StopAsyncIteration'
updated_at = <Date 2017-03-31.16:36:18.956>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:18.956>
actor = 'dstufft'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2016-11-16.23:26:54.874>
closer = 'yselivanov'
components = ['Interpreter Core']
creation = <Date 2016-11-16.23:12:03.626>
creator = 'yselivanov'
dependencies = []
files = ['45514']
hgrepos = []
issue_num = 28721
keywords = ['patch']
message_count = 2.0
messages = ['281008', '281009']
nosy_count = 3.0
nosy_names = ['ned.deily', 'python-dev', 'yselivanov']
pr_nums = ['552']
priority = 'release blocker'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28721'
versions = ['Python 3.6', 'Python 3.7']

[1st1]

1. aclose() should not propagate StopAsyncIteration:

  async def agen():
     try:
        yield
     except: 
        pass
  gen = agen()
  await agen.asend(None)
  await agen.aclose()  # <- this shouldn't raise StopAsyncIteration

2. athrow() should propagate StopAsyncIteration when the exception was swallowed by the generator and it returned:

  async def agen():
     try:
        yield
     except: 
        pass
  gen = agen()
  await agen.asend(None)
  await agen.athrow(ValueError)  # <- this should raise StopAsyncIteration

[python-dev]

New changeset 0f12a1d3a737 by Yury Selivanov in branch '3.6':
Issue bpo-28721: Fix asynchronous generators aclose() and athrow()
https://hg.python.org/cpython/rev/0f12a1d3a737

New changeset 22cd78026ad1 by Yury Selivanov in branch 'default':
Merge 3.6 (issue bpo-28721)
https://hg.python.org/cpython/rev/22cd78026ad1