fix dependency issue

tox requires pluggy<1.0, which matches pluggy==0.7.1, but pytest
requires pluggy<0.7. Hack by changing the order of parsing dependencies

Author: wwwjfy

requirements_dev.txt

@@ -14,13 +14,13 @@ quart==0.6.4;python_version>="3.6"
 # tests
 coverage==4.5.1
 flake8==3.5.0
-tox==3.1.2
 pytest==3.6.3
 pytest-runner==4.2
 pytest-asyncio==0.8.0
 pytest-tornado==0.5.0
 pytest-mock==1.10.0
 pytest-cov==2.5.1
+tox==3.1.2
 
 # docs
 Sphinx==1.7.5