New Python2 compatibility module for built in functions
Use the new Bio._py3k.builtins module
Remove hack for Python issue 19111, using Bio._py3k.builtins instead.

We've replaced 'from future_builtins import x' with
'from Bio._py3k.builtins import x' instead.
Remove an xrange doctest to avoid 2to3 change
Replace xrange with range from Bio._py3k.builtins
Tweaks so we can drop the 2to3 xrange fixer
Tweaks under Bio/* so we can drop the 2to3 xrange fixer
We don't need the 2to3 xrange fixer now
Define basestring in Bio._py3k.builtins
Disable 2to3 basestring fixer, will use Bio._py3k.builtins instead
Use basestring from Bio._py3k.builtins

The goal here is to be able to use the old Python 2
style 'basestring' under both Python 2 and Python 3,
and thus avoids needing to use the 2to3 basestring
fixer, bringing us closer to not needing 2to3 at all.