input: Write a python function to fetch the content of a given URL and save it to a file named 'page.html'.
  assert fetch_and_save('http://example.com') is None
  assert open('page.html', 'r').read() == requests.get('http://example.com').text
output: def fetch_and_save(url):
      import requests
      response = requests.get(url)
      with open('page.html', 'w') as file:
          file.write(response.text)