Define unicode in Bio._py3k
Use unicode from Bio._py3k
Drop Python 3.2
Don't need 2to3 unicode fixer (can use unicode literals on Python 3.3+)
Avoid needing 2to3 nonzero fixer by defining __bool__

By defining both __nonzero__ (for Python 2) and __bool__
(for Python 3) we avoid needing the 2to3 nonzero fixer,
and a runtime choice of method name.

The redundant method seems harmless.
Don't need to call 2to3 anymore
Talk about native support for Python 3 in NEWS & README
Update Python 3 testing for TravisCI