Sanitize the Python 2 check.
Sanitize the Python 2 check.

This broke for me on a project that has a module called 'builtins'.
Let's just do a proper check using the normal `sys.version_info` API.

pyflakes/pyflakes,epsy/pyflakes,nikolas/pyflakes,asmeurer/pyflakes,jayvdb/pyflakes,bitglue/pyflakes,PyCQA/pyflakes