input: Write a python function to retrieve the latest top 50 news headlines from The Guardian using BeautifulSoup.
  assert len(retrieve_guardian_headlines())==50
  assert isinstance(retrieve_guardian_headlines(), list)
  assert all(isinstance(headline, str) for headline in retrieve_guardian_headlines())
output: def retrieve_guardian_headlines():
      import requests
      from bs4 import BeautifulSoup
  
      url = "https://www.theguardian.com/international"
      response = requests.get(url)
      soup = BeautifulSoup(response.content, 'html.parser')
  
      headlines = []
      for headline in soup.select('.fc-item__title'):
          if len(headlines) < 50:
              headlines.append(headline.get_text(strip=True))
          else:
              break
  
      return headlines