* Use test_utils.run_briefly() to execute pending calls to really close
  transports
* sslproto: mock also _SSLPipe.shutdown(), it's need to close the transport
* pipe test: the test doesn't close explicitly the PipeHandle, so ignore
  the warning instead
* test_popen: use the context manager ("with p:") to explicitly close pipes

Set the _read_fut attribute to None after cancelling it.

This change should fix a race condition with
_ProactorWritePipeTransport._pipe_closed().

Check if the _sock attribute is None to check if the transport is closed.

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

transport at subprocess exit.

Clear also its reference to the transport.

Don't call immediatly self._process_write_backlog() but schedule the call using
call_soon(). _on_handshake_complete() can be called indirectly from
_process_write_backlog(), and _process_write_backlog() is not reentrant.

StreamWriter now raises an exception if it is closed: write(), writelines(),
write_eof(), can_write_eof(), get_extra_info(), drain().

Patch by Marc Abramowitz.

* 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.

waiter is cancelled before setting its exception.

Use new OSError exceptions, factorize stdin.write() code.

cancelled before setting its exception.

* Add unit tests for this case.
* Cleanup also sslproto.py

"_Atomic void*" types work. Change needed on FreeBSD 10 where stdatomic.h is
available but the compiler fails on "_Atomic void*" with "_Atomic cannot be
applied to incomplete type 'void'".

* If an exception is raised during the creation of a subprocess, kill the
  subprocess (close pipes, kill and read the return status). Log an error in
  such case.
* Fix SubprocessStreamProtocol.connection_made() to handle cancelled waiter.
  Add unit test cancelling subprocess methods.