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.

Lib/test/test_contextlib_async.py

@@ -1,5 +1,5 @@
 import asyncio
-from contextlib import asynccontextmanager, AbstractAsyncContextManager, AsyncExitStack
+from contextlib import aclosing, asynccontextmanager, AbstractAsyncContextManager, AsyncExitStack
 import functools
 from test import support
 import unittest
@@ -279,6 +279,63 @@ class AsyncContextManagerTestCase(unittest.TestCase):
             self.assertEqual(target, (11, 22, 33, 44))
 
 
+class AclosingTestCase(unittest.TestCase):
+
+    @support.requires_docstrings
+    def test_instance_docs(self):
+        cm_docstring = aclosing.__doc__
+        obj = aclosing(None)
+        self.assertEqual(obj.__doc__, cm_docstring)
+
+    @_async_test
+    async def test_aclosing(self):
+        state = []
+        class C:
+            async def aclose(self):
+                state.append(1)
+        x = C()
+        self.assertEqual(state, [])
+        async with aclosing(x) as y:
+            self.assertEqual(x, y)
+        self.assertEqual(state, [1])
+
+    @_async_test
+    async def test_aclosing_error(self):
+        state = []
+        class C:
+            async def aclose(self):
+                state.append(1)
+        x = C()
+        self.assertEqual(state, [])
+        with self.assertRaises(ZeroDivisionError):
+            async with aclosing(x) as y:
+                self.assertEqual(x, y)
+                1 / 0
+        self.assertEqual(state, [1])
+
+    @_async_test
+    async def test_aclosing_bpo41229(self):
+        state = []
+
+        class Resource:
+            def __del__(self):
+                state.append(1)
+
+        async def agenfunc():
+            r = Resource()
+            yield -1
+            yield -2
+
+        x = agenfunc()
+        self.assertEqual(state, [])
+        with self.assertRaises(ZeroDivisionError):
+            async with aclosing(x) as y:
+                self.assertEqual(x, y)
+                self.assertEqual(-1, await x.__anext__())
+                1 / 0
+        self.assertEqual(state, [1])
+
+
 class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
     class SyncAsyncExitStack(AsyncExitStack):
         @staticmethod