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 is unsuccessful, 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://example.com")!=None
  assert read_url("https://this-site-does-not-exist.com")==None
output: def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None