pin pytest-asyncio to 0.10.0
The recently released 0.11.0 requires pytest 5+, which doesn't support
Python 2.7
@@ -44,7 +44,7 @@ pep8
pytz
-pytest-asyncio ; python_version >= '3.5'
+pytest-asyncio==0.10.0 ; python_version >= '3.5'
asynctest==0.12.2 ; python_version >= '3.5'
cachetools==2.0.1 ; python_version == '2.7'
