tox: unset extras with non-default factors (#831)

"extras" gets inherited from "testenv".

Author: blueyed

tox.ini

@@ -56,13 +56,15 @@ commands =
     coverage: coverage xml
 
 [testenv:checkqa]
+extras =
 deps =
     flake8
 commands =
     flake8 --version
     flake8 --statistics {posargs:pytest_django pytest_django_test tests}
 
 [testenv:doc8]
+extras =
 basepython = python3.6
 skip_install = true
 deps =
@@ -77,6 +79,7 @@ extras = docs
 commands = sphinx-build -n -W -b html -d docs/_build/doctrees docs docs/_build/html
 
 [testenv:readme]
+extras =
 basepython = python3.5
 deps =
     readme_renderer