input: Write a python function to fetch HTML content from a given URL using urllib.
  assert fetch_html('https://www.example.com').startswith('<!doctype html>')
  assert 'Example Domain' in fetch_html('https://www.example.com')
  assert fetch_html('https://www.google.com').startswith('<!doctype html>')
output: def fetch_html(url):
      import urllib.request
  
      with urllib.request.urlopen(url) as response:
          html = response.read().decode('utf-8')
      return html