nth root of x is x^(1/n), so you can do 9**(1/2) to find the 2nd root of 9, for example. In general, you can compute the nth root of x as:
x**(1/n)
Note: In Python 2, you had to do 1/float(n) or 1.0/n so that the result would be a float rather than an int. For more details, see Why does Python give the “wrong” answer for square root?