[conan center] disallow six

[SSE4]

/cc @uilianries @jgsogo there is some probability, that in the future conan might remove six dependency. there are some existing recipes (example https://github.com/conan-io/conan-center-index/pull/1060/files) using six (for StringIO, urllib, etc.). we might disallow the following statements in the hook:

import six

from six import

from six.<whatever> import