Use six.moves.builtins in Bio.bgzf

This reduces the 2to3 changes, but does not address
the docstrings in Bio.bgzf yet.
Already using Python 3 style map here, don't need itertools.imap
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.