input: Write a function to read the contents from a given URL and return the content as a string if the request is successful. If the request fails, return None. The function should use the requests library.
  assert read_url("https://www.example.com")!=None
  assert read_url("https://www.non_existent_url.com")==None
output: import requests
  
  def read_url(url):
      try:
          response = requests.get(url)
          response.raise_for_status()
          return response.text
      except requests.exceptions.RequestException:
          return None