input: Write a python function to extract headers (h1 to h6) from a given URL and return them as a list.
  assert extract_headers('https://www.example.com/') == ['Example Domain']
  assert extract_headers('https://www.wikipedia.org/') == ['Wikipedia', 'The Free Encyclopedia', 'Languages', 'English', 'Español', '日本語', 'Deutsch', 'Русский', 'Français', 'Italiano', 'Português', 'Polski', '한국어', 'العربية', 'Tiếng Việt', '中文', 'Bahasa Melayu', 'Bahasa Indonesia']
output: def extract_headers(url):
      import requests
      from bs4 import BeautifulSoup
  
      response = requests.get(url)
      soup = BeautifulSoup(response.content, 'html.parser')
      
      headers = []
      for i in range(1, 7):
          headers.extend([header.get_text().strip() for header in soup.find_all(f'h{i}')])
      
      return headers