input: Write a function to initialize a Tweepy API object using given consumer keys and access tokens.
  assert isinstance(initialize_api('consumer_key', 'consumer_secret', 'access_token', 'access_token_secret'), tweepy.API)
  assert initialize_api('invalid_key', 'invalid_secret', 'invalid_token', 'invalid_token_secret') is None
output: import tweepy
  
  def initialize_api(consumer_key, consumer_secret, access_token, access_token_secret):
      try:
          auth = tweepy.OAuth1UserHandler(consumer_key, consumer_secret, access_token, access_token_secret)
          api = tweepy.API(auth)
          api.verify_credentials()
          return api
      except:
          return None