Sync oslo-incubator python3 changes

Syncs these commits:

  0c54b72 python3: use 'as' syntax for exception assignment
  fde1e15 Convert unicode for python3 portability

Note this adds a dependency on the 'six' library.

Change-Id: Ibcffa3ab10f6969f1f088b1f5a890f4aa655e9bf
Merge "Sync oslo-incubator python3 changes"