windows installation: require six

content/install.rst

@@ -309,10 +309,11 @@ provided by Christoph Gohlke.  To install Kwant on Windows
    <http://www.lfd.uci.edu/~gohlke/pythonlibs/#numpy>`_, `SciPy
    <http://www.lfd.uci.edu/~gohlke/pythonlibs/#scipy>`_, `Matplotlib
    <http://www.lfd.uci.edu/~gohlke/pythonlibs/#matplotlib>`_, `Nose
-   <http://www.lfd.uci.edu/~gohlke/pythonlibs/#nose>`_, `Tinyarray
+   <http://www.lfd.uci.edu/~gohlke/pythonlibs/#nose>`_, `Six
+   <http://www.lfd.uci.edu/~gohlke/pythonlibs/#six>`_, `Tinyarray
    <http://www.lfd.uci.edu/~gohlke/pythonlibs/#tinyarray>`_, and `Kwant
    <http://www.lfd.uci.edu/~gohlke/pythonlibs/#kwant>`_ itself.  Make sure to
-   put the downloaded files into a directory with no other ``.whl`` files.
+   put the downloaded files into a directory without any other ``.whl`` files.
 
 7. Open a command prompt with administrator rights, as described in “How do I
    run a command with elevated permissions” at the `Microsoft Windows website