update pytest-cov for pytest 4.1.0 compatibility

Author: wwwjfy

requirements_dev.txt

@@ -20,7 +20,7 @@ pytest-runner==4.2  # pyup: update minor
 pytest-asyncio==0.10.0  # pyup: update minor
 pytest-tornado5==2.0.0  # pyup: update minor
 pytest-mock==1.10.0  # pyup: update minor
-pytest-cov==2.6.0  # pyup: update minor
+pytest-cov==2.6.1  # pyup: update minor
 
 # docs
 Sphinx==1.8.1  # pyup: update minor