input: Write a python function to fetch and print tweets containing a specific keyword using the tweepy library.
  assert fetch_tweets('python') is None
output: def fetch_tweets(keyword):
      import tweepy
  
      # Add your own credentials here
      consumer_key = 'YOUR_CONSUMER_KEY'
      consumer_secret = 'YOUR_CONSUMER_SECRET'
      access_token = 'YOUR_ACCESS_TOKEN'
      access_token_secret = 'YOUR_ACCESS_TOKEN_SECRET'
      
      # Authenticate to Twitter
      auth = tweepy.OAuth1UserHandler(consumer_key, consumer_secret, access_token, access_token_secret)
      api = tweepy.API(auth, wait_on_rate_limit=True)
  
      # Fetch tweets containing the keyword
      for tweet in tweepy.Cursor(api.search_tweets, q=keyword, lang="en").items(10):
          print(f"{tweet.user.name}:{tweet.text}\n")