We don't need this in a Python 3-only world.
Remove six in follows:
1. lower-constraints.txt
2. requirements.txt
3. zaqar/bench/observer.py
4. zaqar/common/api/api.py
5. zaqar/common/api/request.py
6. zaqar/common/api/response.py
7. zaqar/common/pipeline.py
8. zaqar/common/storage/select.py
Change-Id: I8669061e00fd929c24e62ce9fd4ba31aab63e0d4