input: Write a function to read the contents from a given URL and return the text if the status code is 200, otherwise return None.
  
  import requests
  
  def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None
  
  assert read_url('https://www.google.com')!=None
  assert read_url('https://www.fakeurlthatdoesnotexist.com')==None
  assert read_url('https://www.example.com/page-that-does-not-exist')==None
output: def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None