See https://github.com/python/asyncio/pull/291 for details.

* Cleanup gather(): use cancelled() method instead of using private Future
  attribute
* Fix _UnixReadPipeTransport and _UnixWritePipeTransport. Only start reading
  when connection_made() has been called.
* Issue #23333: Fix BaseSelectorEventLoop._accept_connection(). Close the
  transport on error. In debug mode, log errors using call_exception_handler()

Do nothing if the transport is already closed. Before it was not possible to
close the transport twice.

* 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

functions:

* add_signal_handler()
* call_at()
* call_later()
* call_soon()
* call_soon_threadsafe()
* run_in_executor()

Fix also the error message of add_signal_handler() (fix the name of the
function).

  Patch by Ludovic.Gasc.

set the blocking mode of a file descriptor (False if the O_NONBLOCK flag is
set, True otherwise). These functions are not available on Windows.

* Tulip issue 194: Don't use sys.getrefcount() in unit tests
* signal.set_wakeup_fd() can now raise an OSError on Python 3.5

Since Python 3.3, the C signal handler writes the signal number into the wakeup
file descriptor and then schedules the Python call using Py_AddPendingCall().

asyncio uses the wakeup file descriptor to wake up the event loop, and relies
on Py_AddPendingCall() to schedule the final callback with call_soon().

If the C signal handler is called in a thread different than the thread of the
event loop, the loop is awaken but Py_AddPendingCall() was not called yet. In
this case, the event loop has nothing to do and go to sleep again.
Py_AddPendingCall() is called while the event loop is sleeping again and so the
final callback is not scheduled immediatly.

This patch changes how asyncio handles signals. Instead of relying on
Py_AddPendingCall() and the wakeup file descriptor, asyncio now only relies on
the wakeup file descriptor. asyncio reads signal numbers from the wakeup file
descriptor to call its signal handler.

environment variable is set

Fix ResourceWarning: create_connection(), create_datagram_endpoint() and
create_unix_server() methods of event loop now close the newly created socket
on error.

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.

DeprecationWarning warnings. create_unix_server() closes the socket on any
error, not only on OSError.

Mention if the error was caused by a read or a write, and be more specific on
the object (ex: "pipe transport" instead of "transport").

Thanks to Vajrasky Kok for discovering some of them.

DeprecationWarning warnings. create_unix_server() closes the socket on any
error, not only on OSError.

Mention if the error was caused by a read or a write, and be more specific on
the object (ex: "pipe transport" instead of "transport").

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: Add close() back to Unix selector event loop, to remove all signal handlers. Should fix buildbot issues.

This should have no effect, it's a "shouldn't happe" case.
Also tidied up some comments.