Update pytest from 4.4.0 to 6.0.0

requirements_dev.txt

@@ -8,6 +8,6 @@ coverage==4.5.1  # pyup: update minor
 Sphinx==2.0.1  # pyup: update minor
 cryptography==2.6.1  # pyup: update minor
 PyYAML==5.1  # pyup: update minor
-pytest==4.4.0  # pyup: update minor
+pytest==6.0.0  # pyup: update minor
 pytest-runner==4.2  # pyup: update minor
 pytest-asyncio==0.10.0  # pyup: update minor