tests for different versions.

tox.ini

@@ -26,7 +26,7 @@ deps =
     -r{toxinidir}/requirements_dev.txt
 ; If you want to make tox run the tests with the same versions, create a
 ; requirements.txt with the pinned versions and uncomment the following line:
-     -r{toxinidir}/requirements.txt
+;     -r{toxinidir}/requirements.txt
 commands =
     pip install -U pip
     pytest --basetemp={envtmpdir}