input: Write a python function to extract all the hyperlinks from a given HTML document using BeautifulSoup.
  assert extract_hyperlinks('<a href="http://example.com">Example</a>') == ['http://example.com']
  assert extract_hyperlinks('<p>No links here</p>') == []
  assert extract_hyperlinks('<a href="http://example.com">Example</a><a href="http://example2.com">Example2</a>') == ['http://example.com', 'http://example2.com']
output: def extract_hyperlinks(html):
      from bs4 import BeautifulSoup
      soup = BeautifulSoup(html, 'html.parser')
      return [a['href'] for a in soup.find_all('a', href=True)]