Remove accidentally added unused import
Tiago updated the thread handling in Bio.PopGen
Handle some (changed) built in functions via Bio._py3k
Use the new Bio._py3k builtins
Remove hack for Python issue 19111, using Bio._py3k.builtins instead.

We've replaced 'from future_builtins import x' with
'from Bio._py3k import x' instead.
Don't need 2to3 itertools and itertools_imports fixers anymore

$ 2to3 --no-diffs -n -w -f itertools -f itertools_imports Bio BioSQL Tests Doc Scripts
RefactoringTool: No files need to be modified.
Remove an xrange doctest to avoid 2to3 change
Replace xrange with range from Bio._py3k
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
Use basestring from Bio._py3k

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.
Disable 2to3 basestring fixer, using Bio._py3k instead
Define Python 3 like input under Bio._py3k
Replace 'raw_input' with Python3 style 'input' via Bio._py3k
Don't need the 2to3 raw_input fixer anymore.

$ 2to3 --no-diffs -n -w -f raw_input Bio BioSQL Tests Doc Scripts
RefactoringTool: No files need to be modified.
Python 3 import fallback for Tkinter in Bio.SeqUtils
Tweak Bio/bgzf.py doctests not to need 2to3
Python 3 fallback for StringIO in doctests