Modernizes tag.py common py2/3 code.  Fixes relative import in _UID_dict
Modernizes remaining files in main code directory.
Modernizes all test suite files.  Fix for dataset.py PropertyError issue with reraise.
Merge branch 'modernize' into dev
Fix tox.ini after merge.  Further comments on 'modernize' merge in this (full) commit message.

Note 'six' module used, but (a 'to do' item) a home-grown compat module could suffice. Also needs another look at the string/unicode compatibility conversions.