Python issue #23208: Add BaseEventLoop._current_handle
Python issue #23208: Add BaseEventLoop._current_handle

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.

overcastcloud/trollius,overcastcloud/trollius,overcastcloud/trollius