* Remove asyncio.selectors and asyncio._overlapped symbols from the
  namespace of the asyncio module
* Replace "from asyncio import selectors" with "import selectors"
* Replace "from asyncio import _overlapped" with "import _overlapped"

asyncio.selectors was added to support Python 3.3, which doesn't have
selectors in its standard library, and Python 3.4 in the same code
base. Same rationale for asyncio._overlapped. Python 3.3 reached its
end of life, and asyncio is no more maintained as a third party
module on PyPI.

* 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

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

available

Since Python 3.5, socket.socketpair() is now also available on Windows.

Make csock blocking before calling the accept() method, and fix also a typo in
an error message.

Before, regrtest failed to remove the temporary test directory because the
process was still running in this directory.

Close the listening socket if sock.bind() raises an exception.

* Issue #159: Fix windows_utils.socketpair()

  - Use "127.0.0.1" (IPv4) or "::1" (IPv6) host instead of "localhost", because
    "localhost" may be a different IP address
  - Reject also invalid arguments: only AF_INET/AF_INET6 with SOCK_STREAM (and
    proto=0) are supported

* Reject add/remove reader/writer when event loop is closed.
* Fix ResourceWarning warnings

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

asyncio: Add support for running subprocesses on Windows with the IOCP event loop (Richard Oudkerk).