input: Write a function to read the contents from a given URL and return the text if the request is successful; otherwise return None.
  assert read_url("https://www.example.com") is not None
  assert read_url("https://non-existent-site.com") is None
output: import requests
  
  def read_url(url):
      try:
          response = requests.get(url)
          response.raise_for_status()
          return response.text
      except requests.RequestException:
          return None