Clean up aiofiles warnings on 3.8 #754
Description
https://travis-ci.org/github/simonw/datasette/jobs/682624476
Lots of warnings like this:
/home/travis/virtualenv/python3.8.0/lib/python3.8/site-packages/aiofiles/threadpool/utils.py:33
/home/travis/virtualenv/python3.8.0/lib/python3.8/site-packages/aiofiles/threadpool/utils.py:33
/home/travis/virtualenv/python3.8.0/lib/python3.8/site-packages/aiofiles/threadpool/utils.py:33: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
def method(self, *args, **kwargs):
/home/travis/virtualenv/python3.8.0/lib/python3.8/site-packages/aiofiles/threadpool/__init__.py:27
/home/travis/virtualenv/python3.8.0/lib/python3.8/site-packages/aiofiles/threadpool/__init__.py:27: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
def _open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None,