On mac, sendfile throws ENOTCONN on a repeated sendfile call if
the connection is closed. Normalize it to behave like other systems.

* bpo-32662: Implement Server.start_serving() and Server.serve_forever()

New methods:

* Server.start_serving(),
* Server.serve_forever(), and
* Server.is_serving().

Add 'start_serving' keyword parameter to loop.create_server() and
loop.create_unix_server().

* Fix typos
* Change warning text
* Add test

* Make ssh_handshake_timeout None by default.
* Raise ValueError if ssl_handshake_timeout is used without ssl.
* Raise ValueError if ssl_handshake_timeout is not positive.

10 seconds by default.

Recent sock.type fix (see bug 32331) makes sock.type checks simpler
in asyncio.

* Convert asyncio/tasks.py to async/await

* Convert asyncio/queues.py to async/await

* Convert asyncio/test_utils.py to async/await

* Convert asyncio/base_subprocess.py to async/await

* Convert asyncio/subprocess.py to async/await

* Convert asyncio/streams.py to async/await

* Fix comments

* Convert asyncio/locks.py to async/await

* Convert asyncio.sleep to async def

* Add a comment

* Add missing news

* Convert stubs from AbstrctEventLoop to async functions

* Convert subprocess_shell/subprocess_exec

* Convert connect_read_pipe/connect_write_pip to async/await syntax

* Convert create_datagram_endpoint

* Convert create_unix_server/create_unix_connection

* Get rid of old style coroutines in unix_events.py

* Convert selector_events.py to async/await

* Convert wait_closed and create_connection

* Drop redundant line

* Convert base_events.py

* Code cleanup

* Drop redundant comments

* Fix indentation

* Add explicit tests for compatibility between old and new coroutines

* Convert windows event loop to use async/await

* Fix double awaiting of async function

* Convert asyncio/locks.py

* Improve docstring

* Convert tests to async/await

* Convert more tests

* Convert more tests

* Convert more tests

* Convert tests

* Improve test

* Drop Python 3.4 code from asyncio

* Fix notes

* Add missing imports

* Restore comment

* Resort imports

* Drop Python 3.4-3.5 specific code

* Drop redunant check

* Fix tests

* Restore _COROUTINE_TYPES order

* Remove useless code

* 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.

Since Python 3.5, socket.socketpair() is also available on Windows,
and so can be used directly, rather than using
asyncio.windows_utils.socketpair().

Patch by Коренберг Марк.

Patch by Vincent Michel.

Patch by INADA Naoki.

Upstream https://github.com/python/asyncio/pull/408 by Ron Frederick.

Upstream https://github.com/python/asyncio/pull/374 by Ron Frederick.

Patch by Vincent Michel.
See also https://github.com/python/asyncio/pull/326

Issue #26509: In fatal error handlers, don't log ConnectionAbortedError which
occur on Windows.

Issue #26604:

* Add a new optional source parameter to _warnings.warn() and warnings.warn()
* Modify asyncore, asyncio and _pyio modules to set the source parameter when
  logging a ResourceWarning warning

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

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

Don't use yield/yield-from in an except block of a generator. Store the
exception and handle it outside the except block.

Issue #23347: send_signal(), kill() and terminate() methods of
BaseSubprocessTransport now check if the transport was closed and if the
process exited.

Issue #23347: Refactor creation of subprocess transports. Changes on
BaseSubprocessTransport:

* Add a wait() method to wait until the child process exit
* The constructor now accepts an optional waiter parameter. The _post_init()
  coroutine must not be called explicitly anymore. It makes subprocess
  transports closer to other transports, and it gives more freedom if we want
  later to change completly how subprocess transports are created.
* close() now kills the process instead of kindly terminate it: the child
  process may ignore SIGTERM and continue to run. Call explicitly terminate()
  and wait() if you want to kindly terminate the child process.
* close() now logs a warning in debug mode if the process is still running and
  needs to be killed
* _make_subprocess_transport() is now fully asynchronous again: if the creation
  of the transport failed, wait asynchronously for the process eixt. Before the
  wait was synchronous. This change requires close() to *kill*, and not
  terminate, the child process.
* Remove the _kill_wait() method, replaced with a more agressive close()
  method. It fixes _make_subprocess_transport() on error.
  BaseSubprocessTransport.close() calls the close() method of pipe transports,
  whereas _kill_wait() closed directly pipes of the subprocess.Popen object
  without unregistering file descriptors from the selector (which caused severe
  bugs).

These changes simplifies the code of subprocess.py.