Removed usage of the py3kcompat module

Like mentioned in I8e0b49cdfbddcf2d44a601ad06be2e75897905fc the
py3kcompat module is deprecated and should not be used.

Updated module apiclient from oslo-incubator to use six instead of
py3kcompat.

Added six >= 1.6.0 as new requirement.

Change-Id: I2253e5a2701ad046f76a8539405baac2eac739f6