BUG: remove six import
closes #4249

@y-p or @jreback can one of u merge when this passes if possible? i won't have access to a computer for the rest of the night :scream: 
 Forgot to say thanks! 
 sorry.....was out last night.....never really sure why `six` was needed...
 i must've lost my mind for some reason i thought it was a dependency. turns out just later versions of dateutil install it
