input: Write a function to read the contents from a given URL. The function should use the requests library to make a GET request to the provided URL. If the request is successful (status code 200), the function should return the text content of the response. Otherwise, it should return None.
  assert read_url("https://www.example.com")!=None
  assert read_url("https://www.invalidurl.com/nonexistent")==None
output: def read_url(url):
      import requests
      response = requests.get(url)
      if response.status_code == 200:
          return response.text
      else:
          return None