Remove dependent module py3kcompat

Module py3kcompat was removed from oslo-incubator, we can use
six directly.

* Sync commit of removing py3kcompat from oslo
* use six replace usage of py3kcompat

Change-Id: I15b9ffb10e2d5765a1ed07dd5fd4e2bf7b21ec49
Closes-Bug: #1280033
Merge "Remove dependent module py3kcompat"