Fix tests

Workaround pytest-asyncio incompatibility with pytest 4.1 [1]

[1] pytest-dev/pytest-asyncio#104

Author: Chih-Hsuan Yen

.travis.yml

@@ -8,7 +8,7 @@ python:
   - "3.7"
   - "nightly"
   - "pypy3.5"
-install: pip install -U $DEPS pytest pytest-asyncio pytest-xdist flaky structlog
+install: pip install -U $DEPS "pytest<4.1" pytest-asyncio pytest-xdist flaky structlog
 script: pytest
 env:
   global: