Remove use of async_generator from tests

[bmerry]

pytest-asyncio 0.15 no longer supports async-generator, and it's not
needed in Python 3.6+ (which is all that gets tested).

Closes #307.

[bmerry]

For some reason the coverage run is getting stuck every time in CI. I'm trying to debug that on a separate branch now.

[coveralls]

Coverage decreased (-0.04%) to 93.509% when pulling bb9a44f on no-async-generator into 2d267e7 on master.

[bmerry]

For some reason the coverage run is getting stuck every time in CI. I'm trying to debug that on a separate branch now.

For whatever reason, running the tests with pytest -v has made the problem disappear. That's something I wanted to do anyway (so that when things do go wrong there is more feedback).

I'm not worried about the decrease in coverage - I Think that's likely to just be because I removed a few lines of code from the test.