Migrate tox.ini to extras = xxx instead of deps = .[testing]

tox.ini

@@ -2,8 +2,9 @@
 envlist=flake8,mypy,py3
 
 [testenv]
+extras =
+  testing
 deps =
-  .[testing]
   pytest-cov
 commands =
   pytest --cov={envsitepackagesdir}/swh/model \
@@ -18,9 +19,9 @@ commands =
   {envpython} -m flake8
 
 [testenv:mypy]
-skip_install = true
+extras =
+  testing
 deps =
-  .[testing]
   mypy
 commands =
   mypy swh