input: Write a function to read the contents from a given URL. The function should return the text content if the request is successful, otherwise it should return None.
  assert read_url('https://example.com')=='<!doctype html>\n<html>\n<head>\n\t<title>Example Domain</title>...'
  assert read_url('https://invalidurl.com')==None
  assert read_url('https://nonexistent.example')==None
output: def read_url(url):
      try:
          import requests
          response = requests.get(url)
          response.raise_for_status()
          return response.text
      except:
          return None