Log 3 slowest tests to make finding those easier

Author: Edvard Majakari

tox.ini

@@ -8,7 +8,7 @@ deps=
     marshmallow3: marshmallow==3.0.0rc6
 
 whitelist_externals=flake8
-commands=py.test --cov-report html --cov hug -n auto tests
+commands=py.test --durations 3 --cov-report html --cov hug -n auto tests
 
 [testenv:py37-black]
 deps=