appveyor.yml: Install python "six" module.

appveyor.yml: Install python "six" module.

It is now required for the build.

Thanks to Alin Gabriel Serdean for helping figure out the correct
solution here.

This adds --diable-pip-version-check because upgrading pip (as
suggested by the error that this option suppresses) causes pip
to fail, which is in turn a bug in pip that manifests only on
Windows: https://github.com/pypa/pip/issues/3383.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>