input: Write a function to read the contents from a given URL. If the request is successful (status code 200), return the content of the URL as a string. If the request fails, return None.
  assert read_url("https://www.example.com")!=""
  assert read_url("https://www.invalidurl.com")==None
output: import requests
  
  def read_url(url):
      try:
          response = requests.get(url)
          if response.status_code == 200:
              return response.text
          else:
              return None
      except requests.RequestException:
          return None