fix false warning by flake8 3.6.0

pycodestyle which is contained in flake8 3.6.0 has an issue with syntax
`await (await async_func())`.
The issue was filed as PyCQA/pycodestyle#811

Author: wwwjfy

tox.ini

@@ -9,7 +9,7 @@ python =
 
 [testenv:flake8]
 basepython=python
-deps=flake8
+deps=flake8==3.5.0
 commands=flake8 gino
 
 [testenv]