input: Write a function to read the contents from a given URL using Python's requests library. If the request is successful (status code 200), return the text content of the response; otherwise, return None.
  
  
  def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None
  
  assert read_url("https://example.com") is not None
  assert read_url("https://nonexistenturl.com") is None
  
output: def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None