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: Ifabce6866cf82f81a33924ea586987388afa589b
Closes-Bug: #1280033
Merge "Remove dependent module py3kcompat"