Fix incorrect Binary Operation Code Generation

[jbremer]

Fixes the following sample code.

import ast
import codegen
import sys

eval(codegen.to_source(ast.parse('sys.stdout.write(("a" + "b")[:1])')))

As this currently eval()'s the following code.

sys.stdout.write('a' + 'b'[:1])

Cheers, Jurriaan