test: downgrade pytest-asyncio

pytest-asyncio 0.11.0 uses different loops for fixtures and the actual
test and also closes event loop in some cases. Downgrade to 0.10.0 until
it's fixed.

pytest-dev/pytest-asyncio#154
pytest-dev/pytest-asyncio#157

ci/requirements.txt

@@ -4,7 +4,7 @@ docutils
 pylint
 sphinx
 codecov
-pytest-asyncio
+pytest-asyncio==0.10.0  # until https://github.com/pytest-dev/pytest-asyncio/issues/157 is fixed
 asynctest
 pyinotify
 psutil