Tox: add pytest section, update flake8 section

tox.ini

@@ -9,10 +9,19 @@ deps =
 commands =
   {envpython} -m pytest {posargs}
 
+[pytest]
+python_files = *test_*.py
+testpaths = test
+addopts = -r fEs
+
 [testenv:flake8]
+basepython = python3
 skip_install = true
 deps =
-  flake8 >= 3.5
+  flake8
 commands =
-  {envpython} -m flake8 -- --exclude=*_Any.py
+  {envpython} -m flake8 {toxinidir} {posargs}
 
+[flake8]
+max-line-length = 120
+exclude = cubicweb_oauth/migration/*,.tox/*,