bpo-41229: Update docs for explicit aclose()-required cases and add contextlib.aclosing() method (GH-21545)

This is a PR to:

 * Add `contextlib.aclosing` which ia analogous to `contextlib.closing` but for async-generators with an explicit test case for [bpo-41229]()
 * Update the docs to describe when we need explicit `aclose()` invocation.

which are motivated by the following issues, articles, and examples:

 * [bpo-41229]()
 * https://github.com/njsmith/async_generator
 * https://vorpus.org/blog/some-thoughts-on-asynchronous-api-design-in-a-post-asyncawait-world/#cleanup-in-generators-and-async-generators
 * https://www.python.org/dev/peps/pep-0533/
 * https://github.com/achimnol/aiotools/blob/ef7bf0cea7af/src/aiotools/context.py#L152

Particuarly regarding [PEP-533](https://www.python.org/dev/peps/pep-0533/), its acceptance (`__aiterclose__()`) would make this little addition of `contextlib.aclosing()` unnecessary for most use cases, but until then this could serve as a good counterpart and analogy to `contextlib.closing()`. The same applies for `contextlib.closing` with `__iterclose__()`.
Also, still there are other use cases, e.g., when working with non-generator objects with `aclose()` methods.

1 files changed, 3 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Library/2020-07-19-20-10-41.bpo-41229.p8rJa2.rst b/Misc/NEWS.d/next/Library/2020-07-19-20-10-41.bpo-41229.p8rJa2.rst
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+++ b/Misc/NEWS.d/next/Library/2020-07-19-20-10-41.bpo-41229.p8rJa2.rst
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+Add ``contextlib.aclosing`` for deterministic cleanup of async generators
+which is analogous to ``contextlib.closing`` for non-async generators.
+Patch by Joongi Kim and John Belmonte.