input: def read_url(url: str) -> str:
      """ This function reads the contents from a given URL and returns it as a string.
      If the request is successful (status code 200), the function returns the content of the URL.
      If the request is not successful, the function returns None.
      >>> read_url("http://example.com")
      '<!doctype html>\n<html>\n<head>\n    <title>Example Domain</title>\n    ...</html>'
      >>> read_url("http://nonexistent.com")
      None
      """
output:     import requests
      response = requests.get(url)
      if response.status_code == 200:
          return response.text
      else:
          return None