Warnings in Python 3.8

[discosultan]

Using backoff under Python 3.8 produces the following warning:

/home/discosultan/.local/lib/python3.8/site-packages/backoff/_async.py:15
  /home/discosultan/.local/lib/python3.8/site-packages/backoff/_async.py:15: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
    return asyncio.coroutine(coro_or_func)

[bgreen-litl]

Thanks for the report. This is now released as version 1.9.0.

[discosultan]

Thanks @bgreen-litl !