input: Write a python function to extract headlines from a given URL using BeautifulSoup.
  assert extract_headlines("http://example.com/") == ["Example Domain", "More Information"]
  assert extract_headlines("http://test.com/") == ["Test Domain", "Test Information"]
output: def extract_headlines(url):
      import requests
      from bs4 import BeautifulSoup
      
      response = requests.get(url)
      soup = BeautifulSoup(response.content, 'html.parser')
      
      headlines = []
      for tag in soup.find_all(['h1', 'h2', 'h3']):
          headlines.append(tag.get_text(strip=True))
      
      return headlines