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

tox.ini

@@ -2,12 +2,13 @@
 envlist=flake8,mypy,py3-{core,db,server}
 
 [testenv]
+extras =
+  testing-core
+  core: logging
+  db: db, testing-db
+  server: http
 deps =
-  .[testing-core]
-  core: .[logging]
-  db: .[db,testing-db]
   db: pifpaf
-  server: .[http]
   cover: pytest-cov
 commands =
   db: pifpaf run postgresql -- \
@@ -32,9 +33,13 @@ commands =
   {envpython} -m flake8
 
 [testenv:mypy]
-skip_install = true
+extras =
+  testing-core
+  logging
+  db
+  testing-db
+  http
 deps =
-  .[logging,db,http,testing-core,testing-db]
   mypy
 commands =
   mypy swh