Moving unittest2 into conditional requires

Unittest2 is only needed for py26 and under. As we want to eventually
be able to make OpenCAFE Python 3 compatible, we need to keep back-
compatiblity requirements on a conditional level.

Change-Id: Id7fc4ede4c1aef84759e72be0fac28c86b1a09bc
Merge "Moving unittest2 into conditional requires"