I don't yet see a viable answer for 2.7 which doesn't require installing additional dependencies and also starts from the file's directory. It's not nice as a single-line solution, but there's nothing wrong with using the standard utilities.
import os
grandparent_dir = os.path.abspath(  
    os.path.join(  
        os.path.join(  
            os.path.dirname(  
                os.path.abspath(__file__)  
            ),
            os.pardir
        ),
        os.pardir
    )
)
print grandparent_dir
And to prove it works, here I start out in ~/Documents/notes just so that I show the current directory doesn't influence outcome. I put the file grandpa.py with that script in a folder called "scripts". It crawls up to the Documents dir and then to the user dir on a Mac.
(testing)AlanSE-OSX:notes AlanSE$ echo ~/Documents/scripts/grandpa.py 
/Users/alancoding/Documents/scripts/grandpa.py
(testing)AlanSE-OSX:notes AlanSE$ python2.7 ~/Documents/scripts/grandpa.py 
/Users/alancoding
This is the obvious extrapolation of the answer for the parent dir. Better to use a general solution than a less-good solution in fewer lines.
     
              
pathlibsolution is a little nicer and more readable, but isn't included with Python 2.7. I'd say stick with what you've got, maybe add a comment.