input: Write a python function to fetch and print the titles of top headlines from the US using the NewsAPI.
  assert fetch_top_headlines() == ['Headline 1', 'Headline 2', 'Headline 3']
  assert fetch_top_headlines() != []
  assert isinstance(fetch_top_headlines(), list)
output: def fetch_top_headlines():
      import requests
  
      api_key = 'your_api_key_here'
      url = 'https://newsapi.org/v2/top-headlines?country=us&apiKey=' + api_key
      
      response = requests.get(url)
      headlines = response.json()['articles']
      
      titles = [article['title'] for article in headlines]
      return titles