Bump up to the six 1.7.x series

The six 1.7.x series brings in some nice adjustments
that users of six can take advantage of, including
but not limited to (the below were selected from
the six changelog and are widely used six features
across openstack):

- Select the implementations of dictionary iterator
  routines at import time for a 20% speed boost.
- Improve add_metaclass, so that it doesn't end up
  inserting another class into the hierarchy.

The full changelog can be found at:

https://bitbucket.org/gutworth/six/src/35f6/CHANGES?at=1.7.0

Change-Id: I80b09c5921ad277e1ec1da47ae718db8b43a8c23
Merge "Bump up to the six 1.7.x series"