input: Write a python function to post a tweet using the Tweepy API.
  assert post_tweet('Test tweet')=='Tweet posted successfully'
  assert post_tweet('Another test tweet')=='Tweet posted successfully'
  assert post_tweet('Tweeting from a function!')=='Tweet posted successfully'
output: def post_tweet(tweet_text):
      import tweepy
  
      # You need to replace these with your own API keys
      api_key = 'YOUR_API_KEY'
      api_secret_key = 'YOUR_API_SECRET_KEY'
      access_token = 'YOUR_ACCESS_TOKEN'
      access_token_secret = 'YOUR_ACCESS_TOKEN_SECRET'
  
      # Authentication with Twitter API
      auth = tweepy.OAuth1UserHandler(api_key, api_secret_key, access_token, access_token_secret)
      api = tweepy.API(auth)
  
      try:
          api.update_status(status=tweet_text)
          return 'Tweet posted successfully'
      except tweepy.TweepError as e:
          return f'Error: {e.reason}'