input: Write a function to read in a tweet and remove all punctuation, special characters, and stopwords from it. The function should return the cleaned list of words.
  assert clean_tweet("I'm so excited to be going to the #MuseumTour! #BigDay #travel")==['excited', 'going', 'MuseumTour', 'BigDay', 'travel']
output: def clean_tweet(tweet):
      import string
      stopwords = set(["i'm", "to", "be", "the"])
      words = tweet.split()
      cleaned_words = []
      for word in words:
          word = word.strip(string.punctuation).lower()
          if word not in stopwords:
              cleaned_words.append(word)
      return cleaned_words