input: Write a python function to generate retweets for a given post_id.
  assert len(generate_retweets(12345)) == 10
  assert isinstance(generate_retweets(12345)[0], str)
  assert 'RT @' in generate_retweets(12345)[0]
output: import random
  
  def generate_retweets(post_id):
      retweets = []
      for i in range(10):
          user_id = random.randint(1000, 9999)
          retweet = f"RT @user{user_id}: This is a retweet for post_id {post_id}"
          retweets.append(retweet)
      return retweets