asyncio: Remove asyncio.timeout() context manager.

It will probably be added back in Python 3.6, once its compatibility
issues are resolved; see [1] for more details.

[1] https://mail.python.org/pipermail/async-sig/2016-June/000045.html

Lib/asyncio/tasks.py

@@ -4,7 +4,6 @@
            'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
            'wait', 'wait_for', 'as_completed', 'sleep', 'async',
            'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
-           'timeout',
            ]
 
 import concurrent.futures
@@ -737,55 +736,3 @@ def callback():
 
     loop.call_soon_threadsafe(callback)
     return future
-
-
-def timeout(timeout, *, loop=None):
-    """A factory which produce a context manager with timeout.
-
-    Useful in cases when you want to apply timeout logic around block
-    of code or in cases when asyncio.wait_for is not suitable.
-
-    For example:
-
-    >>> with asyncio.timeout(0.001):
-    ...     yield from coro()
-
-
-    timeout: timeout value in seconds or None to disable timeout logic
-    loop: asyncio compatible event loop
-    """
-    if loop is None:
-        loop = events.get_event_loop()
-    return _Timeout(timeout, loop=loop)
-
-
-class _Timeout:
-    def __init__(self, timeout, *, loop):
-        self._timeout = timeout
-        self._loop = loop
-        self._task = None
-        self._cancelled = False
-        self._cancel_handler = None
-
-    def __enter__(self):
-        self._task = Task.current_task(loop=self._loop)
-        if self._task is None:
-            raise RuntimeError('Timeout context manager should be used '
-                               'inside a task')
-        if self._timeout is not None:
-            self._cancel_handler = self._loop.call_later(
-                self._timeout, self._cancel_task)
-        return self
-
-    def __exit__(self, exc_type, exc_val, exc_tb):
-        if exc_type is futures.CancelledError and self._cancelled:
-            self._cancel_handler = None
-            self._task = None
-            raise futures.TimeoutError
-        if self._timeout is not None:
-            self._cancel_handler.cancel()
-            self._cancel_handler = None
-        self._task = None
-
-    def _cancel_task(self):
-        self._cancelled = self._task.cancel()