input: Write a python function to fetch data from a given URL.
  assert fetch_data('http://www.example.com').startswith('<!doctype html>')
  assert fetch_data('http://www.example.com').endswith('</html>')
  assert 'Example Domain' in fetch_data('http://www.example.com')
output: def fetch_data(url):
      import requests
      response = requests.get(url)
      return response.text