Fix linting issue (#22)

... which arise with the new flake8 version.

`tests` is no longer a valid flake8 param, as it is no top level
directory.

As it is a subfolder of the `src` folder, it gets linted anyway.

modified:   tox.ini

tox.ini

@@ -41,7 +41,7 @@ commands_pre =
 commands =
     isort --check-only --diff --recursive {toxinidir}/src setup.py
     - flake8 --format=html src setup.py
-    flake8 src tests setup.py
+    flake8 src setup.py
 deps =
     isort
     flake8