input: Write a function to process movie reviews and build a feature set by removing stopwords and using a bag of words.
  assert process_data("This is an amazing movie!") == ['amazing', 'movie']
  assert process_data("The plot was dull and uninteresting.") == ['plot', 'dull', 'uninteresting']
  assert process_data("Great performances by the cast!") == ['great', 'performances', 'cast']
output: def process_data(review):
      stopwords = {'the', 'is', 'and', 'a', 'an', 'was', 'by', 'this', 'of', 'to', 'it', 'that'}
      words = review.lower().split()
      feature_set = [word.strip('.,!?') for word in words if word not in stopwords]
      return feature_set