sklearn >= 0.22.0
sklearn.metrics has a mean_squared_error function with a squared kwarg (defaults to True). Setting squared to False will return the RMSE.
from sklearn.metrics import mean_squared_error
rms = mean_squared_error(y_actual, y_predicted, squared=False)
sklearn < 0.22.0
sklearn.metrics has a mean_squared_error function. The RMSE is just the square root of whatever it returns.
from sklearn.metrics import mean_squared_error
from math import sqrt
rms = sqrt(mean_squared_error(y_actual, y_predicted))