input: Write a python function to read a URL and return its content if the status code is 200, otherwise return None.
  assert read_url('https://httpbin.org/status/200') is not None
  assert read_url('https://httpbin.org/status/404') is None
  assert read_url('https://httpbin.org/status/500') is None
output: def read_url(url):
      import requests
      try:
          response = requests.get(url)
          if response.status_code == 200:
              return response.text
          else:
              return None
      except requests.RequestException:
          return None