input: Write a python function to check if a website is accessible and return its content.
  assert check_website('https://www.example.com') is not None
  assert check_website('https://www.nonexistentwebsite.com') is None
output: import requests
  
  def check_website(url):
      try:
          response = requests.get(url)
          if response.status_code == 200:
              return response.text
          else:
              return None
      except requests.RequestException:
          return None