input: Write a function to read the contents from a given URL. If the request is successful (status code 200), return the content as text; otherwise, return None.
  assert read_url("https://example.com") is not None
  assert read_url("https://invalidurl1234567890.com") is None
output: import requests
  
  def read_url(url):
      response = requests.get(url)
      if response.status_code == 200:
          return response.text
      else:
          return None