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')=='<!doctype html>\n<html>\n<head>\n    <title>Example Domain</title>...'
  assert read_url('https://www.invalidurl.com')==None
output: import requests
  
  def read_url(url):
      try:
          response = requests.get(url)
          response.raise_for_status()
          return response.text
      except requests.RequestException:
          return None