Patch by Philip Jenvey

Patch by Chris Angelico (issue #26221)

See https://github.com/python/asyncio/pull/292 for details.

* PipeHandle now uses None instead of -1 for a closed handle
* Sort imports in windows_utils.
* Fix test_events on Python older than 3.5. Skip SSL tests on the
  ProactorEventLoop if ssl.MemoryIO is missing
* Fix BaseEventLoop._create_connection_transport(). Close the transport if the
  creation of the transport (if the waiter) gets an exception.
* _ProactorBasePipeTransport now sets _sock to None when the transport is
  closed.
* Fix BaseSubprocessTransport.close(). Ignore pipes for which the protocol is
  not set yet (still equal to None).
* TestLoop.close() now calls the close() method of the parent class
  (BaseEventLoop).
* Cleanup BaseSelectorEventLoop: create the protocol on a separated line for
  readability and ease debugging.
* Fix BaseSubprocessTransport._kill_wait(). Set the _returncode attribute, so
  close() doesn't try to terminate the process.
* Tests: explicitly close event loops and transports
* UNIX pipe transports: add closed/closing in repr(). Add "closed" or "closing"
  state in the __repr__() method of _UnixReadPipeTransport and
  _UnixWritePipeTransport classes.

* Fix pyflakes warnings: remove unused imports and variables
* asyncio.test_support now uses test.support and test.script_helper if available

release mode

* _WaitHandleFuture.cancel() now notify IocpProactor through the overlapped
  object that the wait was cancelled.
* Optimize IocpProactor.wait_for_handle() gets the result if the wait is
  signaled immediatly.
* Enhance representation of Future and Future subclasses

  - Add "created at filename:lineno" in the representation
  - Add Future._repr_info() method which can be more easily overriden than
    Future.__repr__(). It should now be more easy to enhance Future
    representation without having to modify each subclass. For example,
    _OverlappedFuture and _WaitHandleFuture get the new "created at" information.
  - Use reprlib to format Future result, and function arguments when formatting a
    callback, to limit the length of the representation.

* Fix repr(_WaitHandleFuture)
* _WaitHandleFuture and _OverlappedFuture: hide frames of internal calls in the
  source traceback.
* Cleanup ProactorIocp._poll(): set the timeout to 0 after the first call to
  GetQueuedCompletionStatus()
* test_locks: close the temporary event loop and check the condition lock
* Remove workaround in test_futures, no more needed

- Issues #21936, #21163: Fix sporadic failures of
  test_future_exception_never_retrieved()
- Handle.cancel() now clears references to callback and args
- In debug mode, repr(Handle) now contains the location where the Handle was
  created.

- Tulip issue 185: Add a create_task() method to event loops. The create_task()
  method can be overriden in custom event loop to implement their own task
  class. For example, greenio and Pulsar projects use their own task class. The
  create_task() method is now preferred over creating directly task using the
  Task class.
- tests: fix a warning
- fix typo in the name of a test function
- Update AbstractEventLoop: add new event loop methods; update also the unit test

of a Future with call_soon(). Add an helper, a private method, to set the
result only if the future was not cancelled.

Handle objects are created. Pass the traceback to call_exception_handler() in
the 'source_traceback' key.

The traceback is truncated to hide internal calls in asyncio, show only the
traceback from user code.

Add tests for the new source_traceback, and a test for the 'Future/Task
exception was never retrieved' log.

- Uniformize repr() output to format "<Class ...>"
- On Python 3.5+, repr(Task) uses the qualified name instead of the short name
  of the coroutine

Use "from unittest import mock". It should simplify my work to merge new tests
in Trollius, because Trollius uses "mock" backport for Python 2.

Thanks to Vajrasky Kok for discovering some of them.

Thanks to Vajrasky Kok for discovering some of them.

Major changes:

- StreamReader.readexactly() now raises an IncompleteReadError if the
  end of stream is reached before we received enough bytes, instead of
  returning less bytes than requested.

- Unit tests use the main asyncio module instead of submodules like events

- _UnixWritePipeTransport now also supports character devices, as
  _UnixReadPipeTransport. Patch written by Jonathan Slenders.

- Export more symbols: BaseEventLoop, BaseProactorEventLoop,
  BaseSelectorEventLoop, Queue and Queue sublasses, Empty, Full

asyncio: Pass cancellation from wrapping Future to wrapped Future. By Saúl Ibarra Corretgé (mostly).