input: def read_url(url):
      """ Read the contents from a given URL. If the URL is valid and the 
      request is successful (status code 200), return the text content of the 
      URL. Otherwise, return None.
      >>> read_url('https://dummyurl.com/valid')
      'Sample content of the valid URL'
      >>> read_url('https://dummyurl.com/invalid')
      None
      """
output:     import requests
      try:
          response = requests.get(url)
          if response.status_code == 200:
              return response.text
          else:
              return None
      except requests.RequestException:
          return None