Added astropy.extern.six to Changes.rst
Merge pull request #1470 from mdboom/unit/fix-error-message-about-array-type

Fix error message about acceptable unit scale data types
Merge pull request #1474 from keflavich/patch-2

Added astropy.extern.six to Changes.rst
Refactor astropy.units to use six instead of 2to3
Encode signatures as byte strings