Asyncio-extras

- Fixed regression on older Python 3.5 releases caused by the introduction of
``_get_running_loop()`` calls

- Fixed ``StopAsyncIteration`` exceptions from leaking from async context managers if ``return``
is used after ``yield``
- Fixed exception being reraised from the context block even if it's handled inside the context
manager function
- Added safeguard to prevent ``call_async()`` from being called from the event loop thread

- Removed the asynchronous generator implementation in favor of Nathaniel J. Smith's
async_generator library. The ``yield_async()``, ``isasyncgenfunction()`` and
``async_generator()`` functions are now deprecated and will be removed in the next major release.

- Renamed the ``isasyncgeneratorfunction`` function to ``isasyncgenfunction`` to match the new
function in the ``inspect`` module in Python 3.6 (the old name still works though)
- Updated ``isasyncgenfunction`` to recognize native asynchronous generator functions in Python 3.6
- Updated ``async_contextmanager`` to work with native async generator functions in Python 3.6
- Changed asynchronous generators to use the updated ``__aiter__`` protocol on Python 3.5.2 and
above
- Added the ability to asynchronously iterate through ``AsyncFileWrapper`` just like with a regular
file object

- Fixed error when throwing an exception into an asynchronous generator when using asyncio's debug
mode

- Fixed concurrency issue with ``async with threadpool()`` when more than one coroutine from the
same coroutine function is being run