tox: activate coverage by default

tox.ini

@@ -4,10 +4,10 @@ envlist=flake8,py3
 [testenv:py3]
 deps =
   -r requirements-test.txt
-  coverage
+  pytest-cov
   pifpaf
 commands =
-  pifpaf run postgresql -- pytest {posargs}
+  pifpaf run postgresql -- pytest --cov=swh --cov-branch {posargs}
 
 [testenv:flake8]
 skip_install = true