Test: update tox configuration

- install deps from dev-requiremnts.txt file
- update pytest section
- update flake8 section

dev-requirements.txt

+https://hg.logilab.org/master/cubes/sioc/archive/default.tar.bz2
+

tox.ini

 [tox]
-envlist = py27,py3,flake8
+envlist = py27, py3, flake8
 
 [testenv]
 deps =
   pytest
-  https://hg.logilab.org/master/cubes/sioc/archive/default.tar.bz2
+  -rdev-requirements.txt
 commands =
   {envpython} -m pytest {posargs}
 
-[testenv:flake8]
-skip_install = true
-deps =
-  flake8
-commands = flake8
+[pytest]
+python_files = *test_*.py
+testpaths = test
+addopts = -r fEs
 
-[testenv:flake8-jenkins]
+[testenv:flake8]
+basepython = python3
 skip_install = true
 deps =
   flake8
-commands = flake8 --exit-zero --show-source --output-file=code_quality.log {toxinidir}
+commands = {envpython} -m flake8 {toxinidir} {posargs}
 
 [flake8]
-format = pylint
 max-line-length = 100
 ignore = E731,W503
 exclude = cubicweb_blog/__pkginfo__.py,cubicweb_blog/migration/*,test/data/*,.tox/*
 
-[pytest]
-python_files = *test_*.py
-testpaths = test
+[testenv:flake8-jenkins]
+skip_install = true
+deps =
+  flake8
+commands = flake8 --exit-zero --show-source --output-file=code_quality.log {toxinidir}