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/
 * ef7bf0cea7/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.

Doc/library/contextlib.rst

@@ -154,6 +154,39 @@ Functions and classes provided:
    ``page.close()`` will be called when the :keyword:`with` block is exited.
 
 
+.. class:: aclosing(thing)
+
+   Return an async context manager that calls the ``aclose()`` method of *thing*
+   upon completion of the block.  This is basically equivalent to::
+
+      from contextlib import asynccontextmanager
+
+      @asynccontextmanager
+      async def aclosing(thing):
+          try:
+              yield thing
+          finally:
+              await thing.aclose()
+
+   Significantly, ``aclosing()`` supports deterministic cleanup of async
+   generators when they happen to exit early by :keyword:`break` or an
+   exception.  For example::
+
+      from contextlib import aclosing
+
+      async with aclosing(my_generator()) as values:
+          async for value in values:
+              if value == 42:
+                  break
+
+   This pattern ensures that the generator's async exit code is executed in
+   the same context as its iterations (so that exceptions and context
+   variables work as expected, and the exit code isn't run after the
+   lifetime of some task it depends on).
+
+   .. versionadded:: 3.10
+
+
 .. _simplifying-support-for-single-optional-context-managers:
 
 .. function:: nullcontext(enter_result=None)

Doc/reference/expressions.rst

@@ -643,6 +643,16 @@ after resuming depends on the method which resumed the execution.  If
 :meth:`~agen.asend` is used, then the result will be the value passed in to
 that method.
 
+If an asynchronous generator happens to exit early by :keyword:`break`, the caller
+task being cancelled, or other exceptions, the generator's async cleanup code
+will run and possibly raise exceptions or access context variables in an
+unexpected context--perhaps after the lifetime of tasks it depends, or
+during the event loop shutdown when the async-generator garbage collection hook
+is called.
+To prevent this, the caller must explicitly close the async generator by calling
+:meth:`~agen.aclose` method to finalize the generator and ultimately detach it
+from the event loop.
+
 In an asynchronous generator function, yield expressions are allowed anywhere
 in a :keyword:`try` construct. However, if an asynchronous generator is not
 resumed before it is finalized (by reaching a zero reference count or by
@@ -654,9 +664,9 @@ generator-iterator's :meth:`~agen.aclose` method and run the resulting
 coroutine object, thus allowing any pending :keyword:`!finally` clauses
 to execute.
 
-To take care of finalization, an event loop should define
-a *finalizer* function which takes an asynchronous generator-iterator
-and presumably calls :meth:`~agen.aclose` and executes the coroutine.
+To take care of finalization upon event loop termination, an event loop should
+define a *finalizer* function which takes an asynchronous generator-iterator and
+presumably calls :meth:`~agen.aclose` and executes the coroutine.
 This  *finalizer* may be registered by calling :func:`sys.set_asyncgen_hooks`.
 When first iterated over, an asynchronous generator-iterator will store the
 registered *finalizer* to be called upon finalization. For a reference example