input: Write a python function to add and get movie ratings using the MovieRatings class.
  assert m = MovieRatings(); m.add_rating('Inception', 9); m.get_rating('Inception') == 9
  assert m = MovieRatings(); m.add_rating('The Matrix', 10); m.get_rating('The Matrix') == 10
  assert m = MovieRatings(); m.get_rating('Avatar') == 'Movie not found.'
output: class MovieRatings:
      def __init__(self):
          self.ratings = {}
  
      def add_rating(self, movie, rating):
          self.ratings[movie] = rating
  
      def get_rating(self, movie):
          return self.ratings.get(movie, 'Movie not found.')