input: Write a function to read and return the contents from a given URL. If the request is successful (status code 200), return the text content. 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.example.com") is not None
  assert read_url("https://www.invalidurlxyz.com") is None
output: def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None