tox: install test deps from setup.py file's [testing] extras

This prevent from requiring a too much recent tox version to fire the
tests, especially this allows to run tests using stretch's tox version.

tox.ini

@@ -3,7 +3,7 @@ envlist=flake8,py3
 
 [testenv:py3]
 deps =
-  -r requirements-test.txt
+  .[testing]
   pytest-cov
 commands =
   pytest --cov=swh --cov-branch {posargs}