* Remove unused SSLProtocol._closing attribute
* test_sslproto: skip test if ssl module is missing
* Python issue #23208: Don't use the traceback of the current handle if we
  already know the traceback of the source. The handle may be more revelant,
  but having 3 tracebacks (handle, source, exception) becomes more difficult to
  read. The handle may be preferred later but it requires more work to make
  this choice.

Patch by Mayank Tripathi.

If ReadFile() fails with ERROR_BROKEN_PIPE, the operation is not pending: don't
register the overlapped.

I don't know if WSARecv() can fail with ERROR_BROKEN_PIPE. Since
Overlapped.WSARecv() already handled ERROR_BROKEN_PIPE, let me guess that it
has the same behaviour than ReadFile().

If UnregisterWaitEx() fais with ERROR_IO_PENDING, it doesn't mean that the wait
is unregistered yet. We still have to wait until the wait is cancelled.

Use a coroutine with asyncio.sleep() instead of call_later() to ensure that the
schedule call is cancelled.

Add also a unit test cancelling connect_pipe().

If a client connected before the server was closed: drop the client (close the
pipe) and exit.

a function or a loop (e.g. "return" or "break").

In debug mode, BaseEventLoop._run_once() now sets the
BaseEventLoop._current_handle attribute to the handle currently executed.
In release mode or when no handle is executed, the attribute is None.

BaseEventLoop.default_exception_handler() displays the traceback of the current
handle if available.

The special case for connect_pipe() is not more needed. connect_pipe() doesn't
use overlapped operations anymore.

Fix create_datagram_endpoint(), connect_read_pipe() and connect_write_pipe():
close the transport if the task is cancelled or on error.

directory containing a backslash.

returned NotImplemented.  Removed incorrect implementations of __ne__().

Patch by Cory Benfield.

replacment string longer than mailformed input data.

This change parallels the language used in the unix man page.

Added a new tunnel test to verify setting of _tunnel_host, _tunnel_port,
_tunnel_headers attributes on HTTPConnection object.

*any* kwarg supplied to _assert_python causes it to not append -E to the
command line flags so without='-E' does effectively work.

script_helper.assert_python_failure().  No such feature has ever existed,
thus it doesn't do what the comment claims.  (It does add a 'without'
variable to the environment of the child process but that was not intended)

mode.  Explicitly remove the PYTHONFAULTHANDLER environment variable before
launching a child interpreter when its presence would impact the test (the
reason -E was being used in the first place).

This enables running the test in an environment where other Python environment
variables must be set in order for things to run (such as using PYTHONHOME to
tell an embedded interpreter where it should think it lives).

Overlapped.ConnectNamedPipe() now returns a boolean: True if the pipe is
connected (if ConnectNamedPipe() failed with ERROR_PIPE_CONNECTED), False if
the connection is in progress.

This change removes multiple hacks in IocpProactor.

Add _overlapped.ConnectPipe() which tries to connect to the pipe for
asynchronous I/O (overlapped): call CreateFile() in a loop until it doesn't
fail with ERROR_PIPE_BUSY. Use an increasing delay between 1 ms and 100 ms.

Remove Overlapped.WaitNamedPipeAndConnect() which is no more used.

cancelled

_WaitCancelFuture futures

only Exception

This change fixes a race conditon related to _WaitHandleFuture.cancel() leading
to Python crash or "GetQueuedCompletionStatus() returned an unexpected event"
logs. Before, the overlapped object was destroyed too early, it was possible
that the wait completed whereas the overlapped object was already destroyed.
Sometimes, a different overlapped was allocated at the same address, leading to
unexpected completition.

_WaitHandleFuture.cancel() now waits until the wait is cancelled to clear its
reference to the overlapped object. To wait until the cancellation is done,
UnregisterWaitEx() is used with an event instead of UnregisterWait().

To wait for this event, a new _WaitCancelFuture class was added. It's a
simplified version of _WaitCancelFuture. For example, its cancel() method calls
UnregisterWait(), not UnregisterWaitEx(). _WaitCancelFuture should not be
cancelled.

The overlapped object is kept alive in _WaitHandleFuture until the wait is
unregistered.

Other changes:

* Add _overlapped.UnregisterWaitEx()
* Remove fast-path in IocpProactor.wait_for_handle() to immediatly set the
  result if the wait already completed. I'm not sure that it's safe to
  call immediatly UnregisterWaitEx() before the completion was signaled.
* Add IocpProactor._unregistered() to forget an overlapped which may never be
  signaled, but may be signaled for the next loop iteration. It avoids to
  block forever IocpProactor.close() if a wait was cancelled, and it may also
  avoid some "... unexpected event ..." warnings.

tests: Remove unused function; inline another function