input: Write a python function to fetch HTML contents from a URL using the requests library.
  assert fetch_html_contents('http://www.example.com').startswith(b'<!doctype html>')
  assert fetch_html_contents('http://www.example.com').find(b'<title>Example Domain</title>') != -1
output: def fetch_html_contents(url):
      import requests
      response = requests.get(url)
      return response.content