Fix incorrect Binary Operation Code Generation

[jbremer]

Fixes the following sample code.

importastimportcodegenimportsyseval(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