Porting Pootle to Python 3

Options

1. Only py3 - drop Python 2 support
2. py2 + py3 - support both 2 and 3
3. py2 + py3 transitional - support both 2 and 3 until 3 is stable then drop 2

Opinions:

1. Dramatic and no transition, hard to maintain code bases. Difficult to backport. Much easier to maintain.
2. Works for those on old machines and new (though does anyone not have py3 support?). Loads of six and stuff to maintain and know.
3. Preferred Nobody is blocked and all are happy until some point we simply remove all py2 support. Give flexibility of when long or short.

Plan of Action

1. Get it running
2. Decisions:
   1. What do with with Unicode literals
3. Land things in this order:
   1. Backports with regression protection
   2. Six related and compatibility changes with py3 testing
4. Remove py2 support when needed