Use six to make it work. While we're not yet able to gate
keystoneclient on py33, we still need keystoneclient to work when
keystoneclient is included in other libraries' test suites. This was
discovered while working on python3 for python-tuskarclient.
Note that together with I65c28896d1551fd537dd7774e87bc479a477a7ab
keystoneclient can be used safely in python-tuskarclient's py33 tests.
Closes-bug: #1239802
Change-Id: Ie6d31dc5fae7e63bb31a5192fa6765fe35dd026a
43e278c8be