input: Write a function to read the contents from a given URL and return the text if the request is successful, otherwise return None.
  assert read_url("https://www.example.com")=="Example Domain<br>This domain is for use in illustrative examples in documents. You may use this domain in literature without prior coordination or asking for permission.<br><br>More information..."
  assert read_url("https://www.nonexistentwebsite.com")==None
  assert read_url("https://www.github.com")==None
  assert read_url("https://www.google.com")==None
  assert read_url("https://www.wikipedia.org")==None
output: import requests
  
  def read_url(url):
      try:
          response = requests.get(url)
          response.raise_for_status()
          return response.text
      except requests.exceptions.RequestException:
          return None